Equity is your share of the pot on average if the hand runs to showdown from this point.
If you have 40% equity, it means you will win about 40% of the time (or win 40% of the pot on average, including ties) against an opponent’s hand or range, given the current board.
You are all-in on the flop with a flush draw. Against Villain’s made hand, you estimate you win about 1 time out of 3 by the river.
Your equity is roughly 33%.