What is Proliferate?
Proliferate is a new ability made in the set Scars of Mirrodin. This is what proliferate does:
You may choose any number of permanents and/or players with counters on them, then give each another counter of a kind already there.
This can get very deadly if your opponent has even one poison counter, or their creatures have -1/-1 counters! And proliferate doesn't just work with only one permanent or player. It's any, and it's your choice .
Suppose on a battlefield, you have Feral Hydra down with seven +1/+1 counters on it. You also have a Soul Stair Expedition down with two quest counters. You have a Ichorclaw Myr down and therefore your opponent has two poison counters. Then, you cast Throne of Geth, a proliferate card. Proliferate needs something to activate it. In the case of Throne of Geth, you must sacrifice an artifact. If you do, you can put a +1/+1 counter on Feral Hydra, a quest counter on Soul Stair Expedition, and a poison counter on your opponent! However, if you have a Poison counter, you do not need to put another poison counter on yourself. After all, you get to choose which permanents and players get more counters.