The Source is a location on the highest server (where Notch resides). It has access to all the servers. It is what keeps Minecraft running and if it is destroyed, Minecraft would crash and endanger the physical world. It is a giant bedrock plain with beacons scattered across it, with each beacon representing a server. It also has one giant beacon in the center, which communicates with all servers. It is the location of the Last Battle of the Source.