It means to liberate. Ex: in the film 28 days later, hippies freed infectious monkeys thus releasing the zombie apocalypse on us all. The hippies liberated the monkeys making them (hippy) liberators. This also can be confused a lot. Liberators help their people or cause usually in forms of freedom. depending on who is doing said liberating they could be considered so or a terrorist.
It's self explanatory, to liberate or free someone or something from whatever is restraining or denying the rights & freedom they once held. In 90-91 coalition forces were sent to liberate the people of Kuwait from the attack & takeover by the the army of Iraq. So we did, unfortunately the coalition forces were stopped from completing the job merely to look like peacemakers. The reality is that we did liberate Kuwait, but we failed to eliminate the threat that Iraq would once again bring to war. The big brass didn't want to step on Arab toes elsewhere, so the term liberators sounded humane. No war is humane, it is simply war, the war liberated Kuwait.