JCS Events in Jazz Jackrabbit 2 are properties of a tile that spawn things that are neither part of the tileset nor animation data, like enemies, springs or ammo. It is currently only possible to place one event on a tile. However, belts and MCEs can be used to work around this limitation.
The four root event categories in JCS are:
There are also some events that don’t appear on this list, such as the Generator event.
For the whole hierarchy, see Hierarchy of JCS Events.
Eat your lima beans, Johnny.