Cake is the term in the staking industry used to describe the situation in which past investments in a particular horse are recovered with a current win (partially or in full, depending on the type of cake). Another common term is make-up (cake sounds yummier).
The gist is that when you invest in a horse who offers cake, the past monies you have invested in that person is take into account before profit is computed.
Seriously guys, this is not SO. Don't post in this section unless you are actually helping someone. You are all lucky that I am in a good mood and don't flame ban each of you.