DeHackEd is an ancient and antiquated means to edit the behavior of items and monsters in Doom. It's severely limited, but most source ports are fully or mostly compatible with it all the way down to vanilla (if you hack the vanilla exe that is, hence the name deHACKed).

The gist is this - Doom monsters, weapons, and items (“Things”) update their behavior every tic of game logic. 35 tics per second. So if you make a rabbit thing and want it to jump up every 2 seconds you set it's jumping “State” to happen every 70 tics. Every state has a set amount of time it runs for before moving onto the next state. With dehacked you can tell the thing what order to run it's states in, and for how long, and you can attach “Actions” to those states. Like firing a fireball or whatever.

The info you need to specify for each state would be:
Sprite
Animation frame of that sprite
The next state this object goes to
Duration of this state in tics (35 per second)
Action this state will execute (fireball, chase, die, etc)

Only certain states can have actions, and once you've used those up that's it. You can't add more with vanilla DeHackEd. Also if you try to use an action state with no action it crashes in vanilla.

WIP - more eventually