JCS, or Jazz Creation Station, is Jazz Jackrabbit 2‘s official level editor. It gives users the capability to make custom levels and tilesets, and edit existing levels (so long as these are not passworded).
The existance of such an editor and the customability it provides to the game has been one of the reasons the community around the game is still vital. It gives the game a lot of replay value, because one can play hundreds of homecooked levels and create new levels using either official or custom tilesets.
JCS used to be called MLLE a long time ago.
See also: Screenshot
Eat your lima beans, Johnny.