Meursault, like the universe, is absurd, in that he defies rational, logical order or comprehensible meaning. We (the readers) never know why he does what he does, thinks the way he thinks, etc. People (whether it is us, the readers, or the characters who encounter him) who attempt to "find" meaning in it/him are absurdists.

In the book, you see countless people "explain" Meursault and his behaviour to themselves in a way that fits with their pre-existing view of the world (the warden and caretaker at his mother's care home, his boss, Raymond, Marie, Salamano, the two lawyers at his trial, the chaplain) but none of these "explanations" are verifiable. All are possible, but none are definitively true.