Gonna quote someone directly rather than paraphrase, but it communicates the idea. (one other way to think of parts of this in US terms would be that removing the "reasonableness" principle would give POTUS Executive Orders - not laws - a lot more power).
Israel's Supreme Court reform crisis | Brookings
The so-called “reasonableness bill” passed with a 64 to 0 vote after all members of the Knesset’s governing coalition voted for it and all members of the opposition left the chamber. Passage of the bill has sparked turmoil in the country. Natan Sachs, director of the Center for Middle East Policy at Brookings and a senior fellow in Foreign Policy, discusses what’s happening in Israel.
SACHS: Well, Israel operates with a common law system. It does not have a clearly defined constitution. It chose instead to have a gradual legislation, what’s known as basic laws, which we can think of as articles of the constitution. But they are very easy to change. And like in most common law systems, especially the British one, a lot of the legislative doctrine comes from history of judicial decisions. And the reasonableness doctrine came about there. It boils down to a judicial review of administrative decisions.
So, reasonableness does not apply to laws. The court will not strike down any law, never struck down a law because it deemed it unreasonable. Rather, it would sometimes evaluate decisions by ministers, the government, or officials, which, for example, could have a conflict of interest or might not have even weighed the serious ramifications of a decision and made something whimsical, capricious decisions which in the United States could also be struck down.
DEWS: So, you wrote a piece, that’s on our website, in February about Prime Minister Benjamin Netanyahu’s efforts to limit the role of the Supreme Court. And you said, quote, “In Netanyahu’s new Israel, the slimmest of majorities could decide anything. Pure, unbridled majoritarianism.” But in this new law, the prime minister says he’s fulfilling the will of the voters. So, what do you make of that claim?
SACHS: So, two points on this. The first is that this issue of reasonableness is just one element. It’s not a small element, but it’s only one element and small in comparison to the vast project that this government set out to limit the ability of the court to oversee or to override both legislative decisions and executive decisions. The goal of the sweeping program by the government was really to undo the one check on legislative and government decisions in Israel. Reasonableness is really only one relatively small element in that.
Why did I write that? Well, if you think of the United States, for example, if a small majority in the House of Representatives wanted to pass an outrageous law curtailing the rights of some minorities, say after a terrorist attack or something else–it’s happened, of course, in history–but you could easily imagine a case like that. Well, it might pass the House perhaps, but it would still have to pass the Senate. And the Senate has a filibuster. Then it would go for a presidential veto, which is hard to override. It can be done, but it’s very hard to override. Then it would go to a whole set of federal courts and they would operate with a very clear Bill of Rights that’s enshrined in the Constitution as amendments to it that are very, very hard to change.
In Israel,if a small majority, say 61 members of 120–or right now 64 members of 120–members of the Knesset wanted to pass a bill curtailing minority rights or individual rights, there is no second House, there’s no higher chamber, there is no presidential veto. There are no courts except only one. The Supreme Court wearing a hat as High Court of Justice that can strike down legislation by the Knesset.
And from The Economist a little more detail on this "reasonableness" doctrine:
By 1989 the city of Jerusalem had been trying for nearly two decades to build a new football stadium. Powerful religious groups who saw matches on the Sabbath as a desecration of Jerusalem’s sanctity had blocked the project, but at last ground could be broken. One obstacle remained. The acting interior minister, Arye Deri, an up-and-coming ultra-Orthodox politician, blocked the land-use change. Teddy Kollek, the city’s mayor, took him to court. In January 1989 the Supreme Court ruled that Mr Deri had acted “in an unreasonable way”.
* Two and a half years later the first match was played at Teddy Stadium.
Legal experts cite the case as one of the earliest examples of the court’s nullifying a government decision on the grounds of “reasonableness”. That principle was introduced in a ruling in 1980 by Aharon Barak, then a new judge on the Supreme Court. It revolutionised Israeli jurisprudence and is a principal source of the constitutional chaos engulfing Israel today.
On July 23rd Binyamin Netanyahu’s government is set to pass an amendment to one of Israel’s quasi-constitutional “basic laws”. It would give the government immunity from the reasonableness standard. Israelis have been taking to the streets since the start of this year in protest.
The reasonableness standard has affected decisions in almost every area of Israeli politics. It was used by the Supreme Court in 1989 to force the army to put on trial a colonel who ordered his soldiers to beat up Palestinian detainees.
* The following year the court invoked it to direct the attorney-general to charge bankers with share manipulation.
* In 1993 the court again took on Mr Deri, ruling that he could not remain interior minister after being indicted for corruption.
* He was forced to resign, convicted and sent to prison. He then returned to politics. In January 2023 the Supreme Court ruled once again he must resign from cabinet because of a conviction for tax fraud.
* Seven of the justices cited the reasonableness standard.
p.s. I would add to it that this site sees fairly frequent questions about the "risks from democracy in action", such as a majority imposing unpleasant rules on minorities (example Q). Almost invariably, they are reassured that Supreme Courts, such as the one at risk of getting neutered here, are the guarantors of this not happening. See also Majoritarianism.
p.p.s. (note the following parallels another answer).
Another risk, very specific to Israel, is due to their electoral setup. The Knesset is heavily fragmented between different parties, with what seems to be often a fluid structure of alliances of convenience, which gives small parties outsized "kingmaker" influence in coalition governments. Let's look at the much-maligned Mr. Deri above. He leads Shas, a fairly right wing Orthodox party with 11/120 parliamentarians. Rather than 61/120 Knesset members imposing something via a bare-minimum majority, a given regulation may very well be an electoral exigency imposed by a small party as a price for its support. So it may not even even represent the will of a significant proportion of Israelis, making it all the more regrettable that there is limited judicial oversight.
* These are cases in which the government could not have been challenged in court by the newly defanged Supreme Court.