The Global Map is the main overview screen that displays all of the locations surrounding your Home Base. This screen allows you to travel to any location that you choose, either by walking, running (at the cost of energy), or riding (must have the Chopper). You can travel out to the Global Map by exiting the edge of any location that you are currently in.
While on the Global Map screen, your Hunger and Thirst bars will not deplete, and you can use items and craft in safety. Additionally, when you exit to the Global Map screen from a location, all hostile NPC Survivors will despawn in addition to any hostile Zombies that have spawned as events (Toxic Spitters and Toxic Abominations - spawned in after arrival - not the zombies that are standing around when you get there).
The Global Map screen also offers the option to Ambush other players, or Build a new Base, none of which have been implemented into the live game yet, but are planned as future features. This screen will also tell you how much Energy you have left, in addition to your current location.