In poker (both online and in real-world games,) pot odds represent the ratio of the current size of the pot to the cost of a call you’ll have to make. Let’s go with a very easy-to-understand example to start: the pot contains $100 and in order to get a shot at that pot, you have to call a minimum bet of $10. This means at that time you have 10:1 pot odds.
Mind you, pot odds are just one part of the probability of winning a hand in order to create an expected value. One of the most common usage of the term is to say that you, as a player “have pot odds,” meaning that your current chances of winning make it profitable to call. The expected value of a hand is determined by a player when they are holding a drawing hand that’s currently behind but is likely to win based on the number of outs available. When the odds of drawing a card that wins the pot are numerically higher than the pot odds, that is a hand with a good expected value.
Let’s use a very basic, exaggerated example. You’re holding Kh10h and the board reads 9hJh and 3s on the flop. You’re one card away from a straight flush (Qh) and three cards away from a straight (Qs, Qd, Qc.) You also have eight other outs (8h, 7h, 6h, 5h, 4h, 3h, 2h and Ah) to get a flush. This means you have something around a 25-28% chance of getting a winning hand on the turn and slightly less (but still favorable) on the river. A $10 bet into a $100 pot is a very favorable call.
On the opposite end, if the odds of drawing a winning hand are numerically lower than the pot odds, the call has a negative expected value and you can, on average, expect to win less money than it costs to call the bet.
Poker pros must be able to instantly determine the expected value of a hand before playing a hand in order to play profitably.