An add-on is a software that improves your game, in the form of a patch for example. Because they knew many bugs and could easily be changed, the Jazz Jackrabbit games known a lot of add-ons. The most commonly used today for these games is certainly JJ2+, very useful for Jazz Jackrabbit 2.
Here are some important add-ons for Jazz Jackrabbit 2.
Eat your lima beans, Johnny.