Slang for the buttocks, the twin cheeks of a person's derriere.
In gay slang, cakes are generally referring to the fleshy cheeks of the buttocks. Icing would be cum, such as in the phrase 'iced his cakes'.
It can also refer to a woman's breasts in straight lingo.
Noteworthy: Some use modifiers to further enhance the mental picture, such as 'sweetcakes' in reference to the cheeks being sweet to taste, to lick, to view, to pump, etc.
Other terms are pancakes, angelcakes, babycakes, honeycakes. Mainly used to show affection, a term of endearment.