At a poker table, to isolate means to make a bet or a raise to get other players to fold and put you in a one-on-one situation against a particular player. In other words, you’re trying to isolate a specific player and play a pot against them heads up.
Players usually resort to this strategy when there is an opponent at the table they perceive as weak or have a specific read on. By isolating that opponent, they get to play them one-on-one without other players messing up their plans.
For example, a player you know is opening any two cards when they enter a pot raises from an early position. Two players call, and it gets to you on the button. You know that the original raiser can have anything, and you want to isolate them and play against them in position. So, you make a big 3-bet from the button with AJ, with the goal of getting the weak player to call and the other two players to fold.
If your plan works, given what you know about this opponent, you’ll be playing them in position with what stands to be the best hand before the flop. Since they’re not a very good player, you’ll also be able to win many pots when you both miss the flop.