Proliferate is a keyword action, meaning that it is an instruction with a very specific meaning like draw, destroy or attach. When an resolving spell or ability instructs you to Proliferate the controller of this spell or ability chooses any number of permanents or players that have at least one counter on them. Then you add one additional counter of that type to these chosen permanents or players. If a permanent or player has more than one counter type on it, then the controller of the Proliferate ability chooses which type of counter to add.