the added pot equity you get from betting based on the chance your opponent folds a better or potentially better hand. its generally used when semibluffing like here.

a turn bet is good because there is the chance that:
a) he calls a worse hand
b) he folds a better or potentially better hand
c) he calls a better hand; but we hit one of our ~7 outs on the river and take down the pot anyway (semi-bluff)

fold equity is an important factor here, because a decent amount of the time he is just going to call us down with a hand like T9 and we arent going to improve to win. we most likely dont have the pot equity with our ace high to make a turn bet a strictly value bet, thus we need the added equity that comes from the times he folds.