Sandbox (abbreviated "SBX") is a freeform game mode with no specific objectives with all players being on the blue team (team 0), and spawning as Builders. Naturally, since players can't normally fight each other (besides with i.e. dirt or dropped spikes, kegs, etc), Sandbox is typically used for building.
Also known as Creative Mode. In KAG Classic, upon joining the server it changes your nickname to "Fat Hobbit Username_Here."