Stack means to pile things on top of each other, creating a vertical arrangement. When you stack books on your desk, you place one on top of another instead of spreading them out. Workers stack boxes in a warehouse, dishwashers stack plates in a cabinet, and you might stack firewood beside the fireplace.

A stack (as a noun) is the pile itself: a stack of pancakes at breakfast, a stack of papers to grade, or a stack of lumber at a construction site. The taller the stack, the more careful you need to be, because stacks can topple if they're unbalanced.

The phrase the deck is stacked against you means conditions are unfairly arranged to make your success difficult, like a card game where someone secretly arranged the cards to help themselves win. Similarly, stacking the deck means arranging conditions to give yourself an advantage.

In computers, a stack refers to a way of organizing information where the last item added is the first one removed, like stacking trays in a cafeteria: you take from the top, and workers add clean trays on top. Programmers use this concept constantly when writing software.