A stack is a tidy pile, like the stack of books on your nightstand or the stack of pancakes on your plate at breakfast. You can also use stack as a verb meaning "to arrange in piles."
If you stack the dollar bills you earned from your lemonade stand, you'll eventually have a stack of dollar bills. Unlike the similar word pile, stack implies a neat, orderly bundle, rather than a messy, haphazard one. When someone "stacks the deck," they cheat in order to achieve the outcome they want—sometimes literally, by arranging a deck of cards unfairly.