CP is the unit of measurment used when refering to how much experience a champion has to determine its level. This can bee seen by the stars at the top. Each star represents the champions level.
To get to this view you can go in to rune manager and right click on a rune individually or you can check the champions level from the list of runes in the rune manager.
To gain CP for a champion there are 2 ways: Every time a champion is deployed in a game it recieves CP at the end of the match or by applying a token.