Definition of pot equity
Your pot equity is, if we make an analogy, your theoretical piece of the pie. Let's take a simple example to illustrate. You are on flop with AT on T-J-5. You are against a single opponent who has a specific range. You evaluate his range and compare it to your hand. Here you give him a 19% rank. Against your hand, his rank will be 43% of pot equity and you, 57%. Theoretically, if the pot was 100€, your pot equity would be 57€ and he would be 43€. Of course, only one player will win 100% of the pot, so that's why we were talking about theoretical fairness.
What's the point of evaluating your pot equity?
Evaluating your pot equity is a way of knowing where you stand in the hand. If, for example, we evaluate our pot equity at 15% and our opponent bets €flop,100 in €100, we clearly know that we can't call the bet. On the other hand, if we think we have 80% of pot equity, it makes it easier. Furthermore, if you identify a situation where your pot equity is 50% against your opponent, it's probably a good time to play your hand aggressively to maximize your chances of getting your opponent to fold when he has a good equity in the hand.