The bubble poker term has many applications, but all refer to the same situation. More precisely, bubble refers to the period just before the money is reached in a tournament setting.
This is when just a few more players need to be eliminated before all other remaining players can reach the cash.
So, for example, let’s say that you’re playing in a tournament that pays money only to the top 40 spots, and the tournament has 500 total entrants. As the tournament winds down, there are only 43 players left at one point.
At this point, it’s said that the tournament is in the bubble phase. The next three players that are eliminated will go home without a cash prize and will have “bubbled” the tournament.