The general understanding of this sentene is that 'you may get something extra' or 'you may get something great'.
In detail, if you know some kinds of 떡, especially 인절미, 콩떡, they have powder made of beans. And the powder is called 떡고물. When I eat 인절미(which is what I ate today), I try to eat 떡고물 as much as possible.
Older people say that sentence to younger ones with other phrases like; 엄마 말만 잘 들으면 자다가도 떡고물이 떨어져. If you only listen to your mom, you will get 떡고물 even when you are sleeping.
If I was not so helpful, somebody else will help you out! ^^