In fact, I believe attempts to create some abstract equalization create a morass of injustices that rebound on the equalizers. Equal justice and equal opportunity are ideals we should seek, but we should recognize that humans administer the ideals and that humans do not have equal ability."

Caveat: my answer's main point is pretty similar to @TheLeopard's excellent previous answer, just paraphrased to be more concise.

What Herbert refers to is the contrast of two political philosophical ideas: "Equality of outcome" and "Equality of opportunity"

Herbert then posits two reasons for his support of the latter over the former:

• Theoretical reason: this is a standard classical liberal/libertarian/conservative argument that, even with the best government efforts, outcomes will never be actually equal. There are many reasons for that, Herbert quotes one of the main ones, namely differences in ability.

  Absent an oppressive totalitarian government, some people will always end up better than others, due to innate ability - grit, willpower, intellect, looks, physical abilities, height, ability to hustle, charisma, even luck. The only way to ensure the equality of outcome is to try to enforce that outcome by brute force, as was done in Socialist states like USSR/China/Venezuela/Mao's China (invariably, wth disastrous results).

  A somewhat simplistic analogy is running - some people will run faster, some people will lose their endurance faster, and the only way to make sure everyone finishes at the same time is forcing faster runners to be delayed by force; or forcing faster runners to start later (again, by force, and denying them equal opportunity).

• Practical reason: Even if you ignore the theoretical reason and decide "no, no, I or someone else smart can figure out a perfect way to administer society that will ensure equal outcome without it becoming a Gulag state", they are ignoring the fact that this perfect society will be administered by humans. Who will end up screwing up this perfection, to either make themselves more equal, paraphrasing Orwell, or even through sheer human fallibility (stupidity, mistakes, lack of foresight, personal issues, interpersonal issues).