Pot odds are the ratio between the current size of the pot and the cost of calling a bet, used to judge whether chasing a drawing hand is worth the price.
The pot holds $100 and an opponent bets $50 on the turn. Calling costs $50 into a pot that becomes $200, for 3-to-1 pot odds, meaning 25 percent equity is needed to break even. A flush draw with nine outs and one card to come is roughly a 4-to-1 underdog, so a plain call loses money here even though the draw hits about one hand in five.
The pot-odds call above is a loser on its own, but raising instead of calling adds a second way to win the pot: the opponent folding outright. That extra chance is fold equity, and it can turn a borderline pot-odds call into a profitable raise even when the drawing hand itself is behind the numbers, see fold-equity for how that second piece works.