Camus' novel is expressive, primarily, of themes relating to absurdity and the difficulty of rendering life meaningful. Mersault is a character who denies conventional and formal explanations of human behavior. He does not see the same reasons for actions nor the same excuses for violence that others do.
In fact, Mersault sees a lack of reason in his most significant act, a murder. When Mersault is captured and imprisoned, many people challenge him to accept their explanations for his behavior. Lawyers and priests provide him with stories to excuse his violence act. Mersault denies them all.
In an expression of Camus’s humanist logic, neither theology nor fate can offer men of intelligence (men like Meursault, willing to use only bare logic to consider the question of life) an explanation for the absolutely senseless things that humans do—war, murder, and other heinous acts. The alternative, therefore, is absurdity.
We can see from early on in the novel that the world does not make sense to Mersault as it does to everyone else. Another way to say this is that Mersault sees absurdity in the world, whereas others see order, sense, and rationality.
For Mersault, reason does not guides men's behavior and certainly does not guide his own when he commits a murder. As a result of this lack of reason, Mersault can draw very little meaning from the act. Where there is no reason and no rationality, there is no meaning.
Articulated as themes, these ideas of absurdity and the difficulty of meaning can also be related to personal inability to construct a meaningful worldview or philosophy. Mersault is a man beset with absurdity.