Just started getting into this game. Bought a rivals box, don’t understand emulate....

thanks in advance

Probably works best when you are drafting, because you can end up with both players having the same copy of a character (Say Luke for example) in play. If there is, you can turn one of the "Imposter Luke" die to any side as there are 2 copies of Luke in play.

Emulate works this way:

when you play it, if there are 2 or more copies of a character in play (yours and your opponent's characters count), you can turn one of those character dice to any side.

Example 1:

You are playing with 2 Executioner characters on your team. You have activated one of them, and rolled a blank. You can play Emulate to turn that blank to any other side for the Executioner die in your pool.

Example 2:

In a draft, you and your opponent are both playing Count Dooku. Your opponent activates his Dooku, and rolls his 3X(R) side. You can play Emulate to turn his Dooku die to the blank.

It's a decent card in draft, because everyone has Anakin, Lobot, Ketsu, and the Jawa Scavenger available. So if you're playing one or two of them, along with other characters that you drafted, the chances are pretty good that you'll face one or two opponents using the same character from the Rivals set.

Otherwise, it's good for non-unique characters like Executioner, as kingbobb mentioned. If you're playing two copies of the same character, you can rig one of their dice, as long as you have the card in hand before one of them is defeated and removed from the game.