An evolution token is used with other currency to rank a player up to a new tier when you are maxed level at your current tier. For example, if you have a player at Bronze level 10, you would use a silver token and currency to bring him to silver level 1.

However, you can only get players to whatever tier progression level you are. So if you are not yet in a silver progression level, the highest you can upgrade your players is bronze level 10.