Dead Land is land that can't be used and needs to be healed first in order to become usable. There's dead land in camp and in levels.
Dead Land is uncovered in camp by removing the fog that's covering it.
There's several ways to heal land:
Each piece of land requires a certain amount of Healing Power to heal. Tapping on the piece of land reveals how much you need.
In levels you can also find super dead land which can only be healed by merging items onto dead land or with a heal extender.