What does it mean when someone says you're a dish?

Assuming I have the context correct, this is a compliment for a female. The compliment may originally have been intended to indicate that all of the girl's qualities are attractive (appearance, personality, intellect, etc.), but it is more often used to compliment only the girl's appearance. This slang dates from the 1940s. While it isn't as common today, it still sees some use.

The term 'dish' is not typically used by itself. There is usually an ambiguous qualifier. "You are quite a dish." or "You are some dish." or "You are a real dish." for example. But, if someone said only that you are a dish, it was probably intended as the same compliment.