The map editor is the main part of QuArK. A map is a 3D world in which you can play; the map editor let's you build your own 3D worlds, and you can use the FPS-game to play in the worlds you built.

Basically, a map is made out of mathematical polyhedrons in 3D space. Each of these polyhedron will be shown in the FPS-game as a wall, floor, ceiling, furniture, or just anything with a fixed 3D shape. Monsters, weapons, and other entities are not made out of polyhedrons, and you cannot use the map editor to build their shape.