Equity
Equity is your share of the pot right now, based on the percent chance your hand will win (or tie) by showdown against what your opponent could be holding.
Equity is a way of measuring how good your hand is as a probability. If you'd win the pot 60% of the time against your opponent's likely cards, you have 60% equity. Multiply that percentage by the size of the pot and you get the chips that, on average, already "belong" to you in that spot, even though the cards aren't done yet.
It comes up most when you're holding a draw or trying to decide whether a call is worth it. Counting your outs (the cards that improve you to the best hand) is the quick way to estimate it: each out is worth roughly 2% per remaining card to come, so an open-ended straight draw with 8 outs has about 16% equity to hit on the next card. You weigh that against the price you're being offered, which ties directly into pot odds.
For beginners, the key idea is that equity is about the long run, not this one hand. Having more equity means you'd come out ahead if the same situation played out a thousand times, so you're not chasing a guarantee. Two related ideas worth knowing by name: fold equity is the extra value you gain when an opponent might fold to your bet, and expected value (EV) folds equity together with the bets to tell you whether a play actually makes chips over time.
Example
You hold A♦ K♦ and the flop is Q♦ 7♦ 2♣, giving you the nut flush draw (9 outs). Against an opponent who already has a pair, you have roughly 35% equity to make your flush by the river, so about a third of the pot is yours on average if you get to see both remaining cards.