As in, what it achieves? Or as in, how it works?
What it achieves: Everytime you evolve a card (it must be a pokemon that can evolve, magikarp can evolve, but Charizard can't), it gets a little stronger.
How it works: You click on evolution when you are on the selection screen, then you select a card of a pokemon that has an evolution in gen 1 or gen 2, after which you hit the blue button to evolve it.