Many good answers already, but the Irish border is a particular sticking point that is not much addressed in existing answers. I will try to explain the Irish border issue in simple terms (but "a few sentences" is impossible). First some history.
Partition of Ireland
The UK has a little brother Ireland. For hundreds of years, the UK was in charge of Ireland (and in fact of a large part of the world, because they had conquered those countries with their powerful army, this is why the US speaks English today). About 100 years ago Ireland declared the independent Republic of Ireland (they had wanted independence much longer). But in Northern Ireland, many people (a bit more than half) felt more British than Irish (because their great-grandparents had (been) moved from Britain, partly to suppress Irish independence) and this part did not become independent. Ireland was, and remains, split. This is called the Partition of Ireland.
Belfast agreement / Good Friday agreement
Many Irish people were very unhappy with this, and there was a war. To resolve this war, the UK and the Republic of Ireland agreed to have similar rules. This way, people in all of Ireland could cross between the Republic and Northern Ireland without noticing the border. Both the ones who wanted (nationalists) and who did not want independence (unionists) were OK with that (at least OK enough to mostly stop fighting). They could have very similar rules because they were together in the EU. This agreement is called the Belfast agreement or the Good Friday Agreement.
Backstop
Now the UK will (most likely) leave the EU but the Republic of Ireland will not. This means the rules will not be similar anymore, and the border between the two will become more visible. Many people in Ireland, in particular republicans in Northern Ireland, are very unhappy about that. They might cross the border twice a day or even have a farm that exists on both sides. The EU and the nationalists wants that Northern Ireland rules stay close to Republic of Ireland rules (so Northern Ireland stays a little in the EU), but the unionists (and many in the UK government) want that Northern Ireland rules stay close to British rules. It is not possible to do both (unless all of UK stays close to EU rules, which is like staying a bit in the EU). The current UK government needs the support from the unionists, so they have to keep them happy. The backstop is a part of the proposed EU-UK temporary agreement (that Parliament rejected) to keep Northern Ireland close to the EU, in case the EU and the UK do not agree a good long term agreement.
Therefore, for the Irish border question alone, it is impossible for the UK to leave the EU while keeping both nationalists and unionists happy.
In the above, I have simplified many things. The history of Ireland is a bit more complicated. Many unionists also do not want a hard border in Ireland (there are two major unionist parties which disagree on the issue of Brexit). The main opposition in the UK (the Labour Party) proposes a solution that they claim avoids the need for a backstop, but not all are sure this is accurate. Some Irish nationalists believe Brexit may be a chance to undo the partition of Ireland and make Ireland one independent country (the idea is that people are so upset with Brexit that they rather join Ireland and EU than stay in a Brexit UK), but that may be wishful thinking for them.
Overall, Brexit is very complicated and cannot be explained in a few child-proof sentences, but I hope this post helps to clarify why the Irish partition is making it harder to find a solution to Brexit.