Philosophy graduate here, I've read the book, and I don't think anyone is too stupid to understand the book, it just requires you to think in a different frame of reference. It's been a long time since I read the book, and a proper analysis would probably run to novel length, but I'll try to summarise if I can.

Meursault is a character very much operating on his own wavelength. He simply doesn't see the merit or meaning of the kind of social and interpersonal structures that other people take so much for granted that it's hard to see past them.

The trial at the end is a really good example of all of this. At first instance, Meursault is French and his victim is Algerian. Politically this means he really does have every possible opportunity to make something up and people will convince themselves that his story is the truth and then he will be set free. He doesn't do this, because it would be inauthentic to do so. He spends his trial concerned with the fact that he's too hot, the possibility that he might be imprisoned, executed even, is nothing he worries about.

The key point here is authenticity. At every point, Meursault is exactly who he is, expresses exactly what he is thinking and feeling, and often that is apposite to the customs (like drinking coffee during the vigil) and social mores of his time (like not protecting himself in the trial even when people are expecting him to do so). When he tells his casual sexual partner that he "probably doesn't" love her, he's being authentic, but it's to his detriment.

What this means is that his immediate concerns are way more important to him than conceptual ones. When he shoots the Algerian, he does so simply because he wants to get some water because it's hot. He doesn't have any higher motive than that. When he's in the prison cell, he manages to be quite happy, remarking that you can get used to anything, because his experience of life expands and contracts with his circumstances without his intervention.

What Meursault is missing is a psychological conception of the future. When we plan, or consider things, we make strategic decisions about how and what we choose to do. We construct rational arguments for why we might do this or that thing, and then use those reasoned constructs to inform behaviour. The problem is that this can be conflicting with our feelings, and to existentialists like Camus or Sartre this was inauthentic because it didn't reflect the reality of what the person was thinking or feeling.

I see this in two ways. Meursault is completely authentic in every way, he never lies, or hides anything from anyone even when it would be to his advantage. He refuses to feel differently, or pretend to feel differently, for the comfort or benefit of anyone else. In this way he is wholly himself in a way other people are not. He is univocal, straightforward, and unsentimental.

On the other hand, he is willfully ignorant of the feelings of his fellow men and women, to the point where he simply is unable to understand why they would need to inform their impression of his character by tertiary sources. He fails completely to grasp the social shorthand of interpersonal interactions, and doesn't use them to his benefit even when it would be moral and sensible to do so.