Your share of the pot right now, expressed as the percentage of the time your hand would end up winning if every remaining card were dealt with no further betting. It is a statement about the current situation, not a prediction about how the hand will be played.
A flush draw on the flop has roughly 36% equity against a made one pair with two cards to come; the same draw with one card to come has about 19%. An overpair against an underpair is around 80/20. A coin-flip between a pair and two overcards is closer to 55/45 than the name suggests.
Equity becomes a decision when you compare it to the price. Pot odds tell you the share you need; equity tells you the share you have. If a call requires 25% and your hand holds 36%, the call profits on this street alone. If it requires 33% and you hold 19%, it does not — unless the money you stand to win on later streets closes the gap, which is a separate calculation and one that assumes your opponent will actually pay you.
Two distinctions matter. Equity against a single hand is not equity against a range, and the range figure is the one your decision is actually against. And raw equity is not the same as equity you will realise: a draw played out of position against an aggressive opponent will be forced to fold some of the time it would have won, so the number on the calculator is an upper bound rather than what you should expect to collect.