Wiktionary
a. 1 (context motor racing English) Holding the last qualifying position with qualification still in progress, and thus liable to lose that position. 2 (context sports English) Having qualification for an event depend on the upcoming performances of other competitors. 3 (context idiomatic English) Uncertain of success.
Wikipedia
To be considered "on the bubble", you must be waiting to find out if a player or team has made the final spots in a tournament or on any given sports team roster. On the bubble is most commonly associated with the Men's college basketball or Women's college basketball for the NCAA Men's Division I Basketball Championship. When you are considered to be on the bubble in the NBA or NFL it means you are being considered for one of the last few positions on the roster. For those NCAA tournament the teams that are on the bubble, their names are usually released by ESPN College Basketball; with the list being updated every few days. The table that ESPN releases consists of eight teams with four of them being in the First Four in and the Last four teams out of the tournament. Many College Basketball teams pay very close attention to the Bubble in late February and early March right before the NCAA tournament begins.
Usage examples of "on the bubble".
I'm less emotionally involved, because I have no relatives on the bubble, but I'm appalled.
God focused all his being on the bubble, searching for a weak point, for a way into the matrix of matter and energy that was displacing him.
Starn nodded in acknowledgment, to more laughter, then broke the seal on the bubble and waited for its field to collapse.
Devra ordered, and springing to her feet she rapped on the bubble, motioning Jarvie to come out.
We were about three deep and Dad had to keep warning people not to lean on the bubble of the stretcher.
There was no symbol on the bubble-tent, or on the other two smaller ones nearer the river.