Dead Land is land that can't be used and needs to be healed first in order to become usable. You can find Dead Land in Camp, Levels and Events.
Dead Land is uncovered in Camp by removing the Evil Fog that covers it.
In Levels and Events you can also encounter Super Dead Land which can only be healed by merging items off it or with a Heal Extender.
How to Heal
Each piece of Dead Land requires a certain amount of Healing Power to heal. Tapping on the piece of land reveals how much you need. There're several ways to heal it: