It protects your skin against the sun's harmful rays.

[Other than that, he doesn't really know, nor does he care to find out. It does its job and that's all that matters.]

Mind returning the bottle? I need to get my back still.

[Besides, he might take a quick swim while he's at the beach. It's been a very long time... years since he's swam in the ocean. Normally he had no time, or would prefer to spend his free time pursuing other interests.]