Isolation plays are those designed to get heads-up against a player over whom you have an advantage. In a positional game such as Texas Hold'em, a player who finds a weak opponent on his right can often isolate by raising or reraising that weak opponent causing all other players to fold. The isolating player can then concentrate on that single opponent. This move often fails in low-mid limit games where cold calling is more common.
The isolation play is usually best used on an opponent over whom you have a lot of control or whose play you understand clearly.