Definition
The bubble in tournament poker refers to the period just before the money — when the next player to bust out will finish out of the money (ITM = in the money). Being "on the bubble" means you are at risk of finishing in the last non-paying position. The player who busts in that spot is said to have "bubbled" the tournament.
Bubble play is one of the most strategically complex stages in tournament poker because ICM pressure is highest — every chip you lose matters disproportionately to your tournament equity. Short stacks are highly incentivized to fold and survive, creating opportunities for chip leaders to "apply bubble pressure" by raising and re-raising frequently.
Experienced tournament players recognize and exploit bubble dynamics: attacking short stacks who are trying to survive, being cautious when facing resistance from stacks that could eliminate you, and understanding when your own stack size gives you leverage or vulnerability.
Example
In a $1,000 tournament paying 50 players, with 51 players left, you are "on the bubble." Short stacks fold extremely tight, trying to survive into the money — creating huge opportunities for chip leaders to steal antes and blinds relentlessly.