Being pot committed describes a situation where the pot is so big that the player can no longer afford to fold to the opponent’s bet, and doing so would be a mathematical mistake.
For example, in a $1/$2 game, after some preflop action, there is $50 in the pot, and you flop a gutshot. You check to your opponent, who continuation bets for $30 and you go for a check-raise to $80. Now they announce an all-in, and it turns out that they only have a total of $90 in their stack, which means that you have to call just $10.
Even though you have one of the weakest possible draws, you’re now pot-committed. You need to call $10 to win $250, and folding here would be a big mistake as long as you have any outs. The only way you could fold is if your opponent turned over their hand and showed you that you had no way to win by the river.
This is why you must always keep track of other players’ stacks to avoid making a play that will accidentally commit you to a pot.