The concept is extremely important. Lots of books and resources available for study on this, including the Poker Strategy Articles here in this forum, accessible from the tab in the top right.
Here's the direct link to the equity article:
https://www.cardschat.com/poker-equity.php
In a nutshell, there are generally two types of equity: showdown equity (often just called equity) and fold equity. Sometimes you'll see fold equity termed "steal equity" but the concept is the same -- the odds of taking down a pot before going to showdown.
Showdown equity is your hand's odds of winning a pot based on the strength of your cards. It is also sometimes stated as your "percentage" of a pot, particularly when talking about all-in expected value (EV). Using the classic example off AA preflop versus any two random cards, the rockets are an 80% favorite to win before any other cards are dealt, so you can say AA has 80% equity in this hand. When you watch the WSOP and see the percentages next to each hand, this is their equity in the pot. Equity between two known hands is a pretty concrete number, but since we never know our opponents' hands, it's easier to put them on a range of hands and consider our equity against their range.
Fold equity refers the likelihood of winning the hand by making your opponents fold, and increases your overall total equity because your opponent may fold the best hand (i.e. the hand with the most showdown equity). This isn't as concrete a value as showdown equity, but rather is more situational and dependent on a lot of variables like stack sizes, table images, scare cards, etc.
Showdown equity is based entirely on the value of your cards against your opponent's cards/range. Fold equity has nothing do with your card values, rather your opponent's perception of your cards or range, or the risk:reward prospect of stacking off against you.
