"We walked on the beach for a long time. By now the sun was overpowering. It shattered into little pieces on the sand and water"
The sun and warmth in this passage is described in a negative way. The sun is too powerful, tiring Meursault out and draining him. He is talking about the sun shattering into little pieces because that is what he feels is happening to him. This can also be taken literally because the sun glistens off the sand and water looking as if it had shattered into little pieces.