Equity
Your share of the pot right now: how often your hand wins if it checks down.
Flop comes Kh-6h-2s. You're in the big blind with 9h-8h, having called a button raise. The button holds Kc-Jd for top pair; you have nothing made, just a flush draw. Four hearts show between your hand and the board, nine more sit in the deck. With two cards to come, nine outs is about 35% equity. Deal the turn and river a hundred times with no more betting and your flush lands in roughly 35 of them, win or lose the rest.
That number tells you whether to call. Villain bets $50 into a $100 pot, so you're risking $50 to win $200 and need 25% to break even. Your 35% clears it with room to spare. That's all pot odds are: what the pot offers weighed against what your hand is worth right now.
The trap is reading 35% as your real chance to win the hand. It assumes the cards run out with no more money going in, which almost never happens — a turn bet and a river bet are still coming, and how much of that 35% you keep depends on position and who holds the betting lead. The button with this same draw collects more of it than you do from the blinds, because acting last lets him see the other player move before he commits another chip. Beginners blow it two ways: freeze the number and forget the betting ahead, or miscount the outs up front. Count a card that pairs the board and hands villain trips as a clean out, and a big draw turns into an expensive one.