A pile is a heap of stuff that keeps accumulating, like the dirty laundry in the back of your closet, or Uncle Scrooge’s money.
Pile can be used as a noun or a verb. If you pile rocks on top of each other, you will eventually have a...pile of rocks. Unlike the word stack, pile implies that your heap of stuff is fairly messy. It can also mean cram. When eight people pile into a car with only five seats, they’re likely to be pretty much squashed.