What does "proliferate" mean on MTG?

Proliferate is a keyword action in Magic: The Gathering. When a player is told to proliferate, that player chooses any number of permanents with a counter on them. Those permanents then receive an additional counter of the same type previously on them.

The player is allowed to select opponents' permanents, Planeswalkers and even other players, including themselves. If a permanent has two or more different types of counters on them, the player who was told to proliferate chooses one of them to add. Also, the player does not have to choose all permanents with counters -- only the ones he or she wishes to add counters to.