Liberation is one of the coolest new features in the game so far. You can liberate several towns and military camps that are marked on the map. Liberating a town means to liberate the town from zombies. So you have to kill all zombies around to complete the liberation.
Additional to this, you have to activate all the generators in the town, before you start killing the zombies. You need to repair and fuel the generators to activate them. Each generator requires one wrench for repairing and a full gas pod to fuel it. You have to do this with all the generators in the town.
You can fuel gas pods if you are near the pump station at a gas station. To fuel a empty gas pod, you need to open your Inventory, right-glick the empty gas-pod and press Refill.
Full Gas Pods won't disappear after using it. They just turn into a empty one. So you can use it again and again.
As soon as you have liberated a town, you'll receive a skin-gift and Supply Drops will land in the liberated town in regular intervals.