A poker bubble most commonly refers to a situation in the tournament that separates the players who make the money from those who walk away with nothing.
For example, if a poker MTT has 1,000 participants and only the top 10% get paid, this means that the 101st player will burst the bubble, as they will be the last player to be eliminated without getting a payout. After that, all 100 players left in this tournament will receive money.
Additionally, this term can also be used as a verb with the same meaning in the context of “bubbling” in a poker tournament. In this sense, the player who bubbled got knocked out just out of the money.