Definition of equity
The equity in a poker hand is the percentage chance that a hand has of winning the prize pool at a given stage. Fairness is not fixed, it depends on many factors. Sometimes we will have an accurate idea of the fairness of a hand and sometimes we will have a less accurate fairness. This is true when we calculate the fairness 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 clarify this.
Some examples of equity
You call an all in preflop with AK out. Your opponent turns over his cards and he has a pair of 7's. From this point on, we will turn over 5 cards (flop + turn + river) and determine which hand will win the pot. With software like Flopzilla for example, it will be possible to calculate the equity of the two hands, i.e. how often each hand will win the prize pool. The software will analyze all possible combinations of the next 5 cards for us and give us the results. Here, AKo will have an equity of 45% and 77 will have an equity of 55%. So 55% of the time, after dealing the 5-card board, 77 will be the best hand. We said earlier that equity is not fixed. To use our AK vs 77 example, if the all in had occurred on the flop on A-7-7, rather than having 55% equity, our pair of 7s would now have 99.9% equity.