Definition of iso-raise
An iso-raise is when you raise preflop (preflop raise) after 1 or more limpers. A limper is a player who simply pays the big blind preflop rather than raising as most players do. So, if the player in the UTG position pays his blind, and a player in the cutoff also limps in, the player on the button can iso-raise the players. This is called an iso-raise because its purpose is to isolate with one player and not to play a multiway hand. The iso-raise will also be larger than a standard raise.
The advantages of iso-raise
If you are the type to raise preflop to 3bb, your iso-raise might be 3bb + 1bb per slider. In other words, to use our example above, 2 players have limped so if the button makes an iso-raise, he will make it 3bb + 2bb so he will raise to 5 big blind. He could have made it bigger if he was out of position. The advantage of the iso-raise is that it will clean up the number of players on the flop. The more players in the hand, the lower our equity will be. By making an iso-raise (bigger than a standard raise), either everyone folds and you win the blinds, or you end up (most of the time) heads-up in position with the initiative (advantageous position).