The price the pot is offering you on a call, expressed as the ratio between what you must put in and what you would be playing to win. It answers exactly one question: how often does this call have to be right to break even?
Work it as a fraction. If the pot holds 100 and someone bets 50, calling costs you 50 and creates a pot of 150. Your call needs to win 50/150 — about 33% of the time — to come out level. Facing a pot-sized bet the number is 33%; a half-pot bet asks for 25%; a quarter-pot bet asks for 20%. Those four numbers cover most of the decisions you will ever face, and they are worth memorising rather than recomputing at the table.
Pot odds are a threshold, not a verdict. They tell you the equity you need; they do not tell you the equity you have. The second half of the calculation is your read: how much of the opponent's betting range do you actually beat, and how many of your outs are genuinely clean?
Two mistakes account for most of the damage. The first is comparing pot odds to how strong your hand feels instead of to a number. Second pair feels weak, but against a range that bluffs often it can be a comfortable call at 4-to-1. The second is forgetting that the price applies to this street only — a flop call that is marginal on pot odds alone may be clearly correct once the money you can win on later streets is counted, and clearly wrong if you will be facing another large bet with no plan.