The amount of money in the pot compared to the amount you must put in the pot to continue playing. For example, suppose there is $60 in the pot. Somebody bets $6, so the pot now contains $66. It costs you $6 to call, so your pot odds are 11:1. If your chance of having the best hand is at least one out of twelve, you should call.
Pot odds also apply to draws. For instance, suppose you have a draw to the nut flush with one card left to come. In this case, you are about a 4:1 underdog to make your flush. If it costs you $8 to call the bet, then there must be at least $32 in the pot (including the most recent bet) to make your call correct.