What is the bubble in a poker tournament?

The bubble is the stretch right before a tournament starts paying out - the money bubble specifically is the single spot where the next player eliminated wins nothing and everyone else left cashes.

Play tightens sharply on the bubble because busting one spot before the money is worth exactly zero, no matter how many chips a player had going in - the entire stack's value collapses to nothing at that one moment. A satellite has its own version of this, the satellite bubble, where the boundary isn't the money but the last seat available.

Pay-jump bubbles also recur throughout a tournament, not just once at the money - every jump to the next payout tier creates the same pressure on a smaller scale.

The pressure is not shared evenly, and that asymmetry is the whole strategic point. A big stack on the bubble risks almost nothing by attacking: it cannot be eliminated by a medium stack, and every fold it forces is chips taken from a player who genuinely cannot afford to call. A medium stack faces the worst version of the problem - enough chips to have real equity to protect, not enough to apply pressure of its own. A very short stack is oddly free, because it is already close to busting and has the least equity left to lose.

That produces the standard bubble dynamic: big stacks open relentlessly, medium stacks fold hands that would be trivial calls an hour earlier, and short stacks pick their one spot. Correctly played, a big stack on a bubble can pick up a meaningful share of the chips in play without ever showing a hand.

The bubble also ends abruptly. The moment the last unpaid player busts, every stack's equity is repriced and correct strategy snaps back toward chip-EV almost immediately. Players who stay in bubble mode for the next two levels give back the entire edge they spent the bubble protecting.