Bubble refers to a situation in a poker tournament where only one player needs to be eliminated before everyone else reaches paid places and is guaranteed to win something.
It is also worth mentioning that a poker bubble can indicate a situation where only one player is left until the final table is formed in the tournament. If that is the case, you will usually hear an adjusted expression of “Final table bubble” rather than just the bubble when someone refers to it.
In both of these cases, the bubble requires significant strategy adjustment since losing your stack in this situation is very costly and will significantly reduce your long-term EV. There are even tools developed to help you understand the independent chip model (ICM) and see how you need to adjust in these situations.
To put it in short, if you have a big stack, you can pressure smaller stacks since they will not want to risk their tournament life in these situations. On the other hand, if you are a small stack yourself, you should try to outwait till the bubble bursts and only then risk your tournament life.