Definition of equity
equity in a poker hand is the percentage chance a hand has of winning the jackpot at a given stage. equity is not fixed, it evaluates according to several factors. Some times we will have an exact idea of equity in a hand and some times we will have a less accurate equity. This is true when we calculate equity of a hand in relation to a range we are evaluating. This evaluation will be less accurate than if we know the exact nature of both hands. Let's take a few examples to make it more precise.
Some examples equity
You call a all in preflop with an unpaired AK. Your opponent turns over his cards and has a pair of 7's. From that point on, we will turn over 5 cards (flop + turn + river) and determine which hand will win the jackpot. With software like flopzilla, for example, it will be possible to calculate equity of the two hands, i.e. how often each hand will win the jackpot. The software will analyze for us all the possible combinations of the 5 cards to come and give us the results. Here, AKo will have a equity of 45% and 77 a equity of 55%. So 55% of the time, after dealing the 5-card table, 77 will be the best hand. We said above that the equity is not fixed. To use our example of AK vs. 77, if the all in would have occurred at the flop on A-7-7, instead of having a 55% equity, our pair of 7s would now have a 99.9% equity.