When two players move all their chips in the middle and have close to equal chances of winning the pot, this situation is considered a coin flip.
The most common example of a coin flip is when one player has overcards against another player’s pocket pair. For example, if you move all in with AKs and are called by pocket jacks, you are considered to be in a coin flip.
As you can guess from the name, it originated from tossing the coin and guessing which side it will land on since this action has an equal chance of going either way. That said, a coin flip in poker is never exactly 50/50, but more in a range of 45/55. If we take the previous example of AKs vs. JJ, you would have around a 46% chance of winning with Ace-King suited and around 54% equity with pocket jacks.