In this sense, `stacked` describes a woman as having large breasts. It is slang, it is about the body, and it is a judgement made by the person looking — usually a man talking about a woman, often to other men. You will hear it in old films, in comedy, in casual talk between friends. The full phrase used to be "well stacked", and older speakers still say it that way; younger speakers just say "stacked". It sounds dated now, and to many people it also sounds crude — not the worst word, but the kind of thing that makes a room go quiet if you say it about someone who can hear you. This is a word to UNDERSTAND, not a word to use. If you meet it in a book or a film, you now know what it means; that is enough.

The same spelling has other, ordinary meanings that have nothing to do with the body. Things put in a pile are stacked ("the plates were stacked by the sink"). A situation that is unfairly arranged is stacked: "the odds are stacked against us" means everything has been piled up on the other side. And in American English, `stacked` can also mean "full of good things" — a stacked sandwich, a stacked line-up of singers at a concert. Which meaning is in play is almost always clear from what is being talked about.