In poker, an isolation play is usually a raise designed to encourage one or more players to fold, specifically for the purpose of making the hand a one-on-one contest with a specific opponent.
For example, if an opponent raises and a player suspects they are holding a weak, but playable hand, they may reraise to pressure other opponents to fold, with the aim of getting heads up with the opening raiser.
However, when a player is extremely short stacked compared to the rest of the field in a tournament, making him bust will sometimes be more profitable than winning his chips, so inducing overcalls from other players trumps isolation play.
[1] Isolating is encouraged when holding a hand that fares better heads up than in a multi-way pot.
For instance, when a player has a small pocket pair they may raise a large amount simply to knock out other players because typically a small pocket pair is about 50–60% likely to win an all-in pot in a heads up situation, but less likely when facing multiple opponents.