Meursault was at the beach house with Raymond and Marie and others. He left to go down to the beach
The sun is barring down on Meursault as he is walking down the beach. He constantly talks about how the sun is very hot and making him agitated.
"Then I fired four more times at the motionless body where the bullets lodged without leaving a trace.”
When Meursault arrives at the beach he sees the Arab laying on the beach in the distance. The Arab is doing nothing to Meursault but Meursault still beings to draw his gun on him.
Meursault then shoots the Arab once. This is absurd because there was no reason for Meursault to shoot the Arab. He kills someone for no reason except for "the hot sun" that he said was burning down on him.
Then Meursault shoots the Arab four more times. This is even more absurd because the Arab is already dead and he keeps shooting him for no reason. He is unecessarily shooting him again. It is even more absurd because he says "it was like knocking on the door of unhappiness four times." He knows what he is doing is extremely wrong but he does not feel any emotion towards his actions.
Meursault says that the sun is what made him shoot the Arab. This is the most absurd thing because he took away a human life just because the hot sun was barring down on him.