I want to know because I have been told that it's a good feature to use in order to optimize large exterior areas.
I've looked everywhere and nobody really explains what multibounds are. Documentation for Construction Kit, Creation kit or GECK just tells you where the button is and doesn't really explain how to use it or how it works.

I noticed that Object References always have a MultiBound tab that you can assign the object to a multibound. But that's pretty much all I know.

That'll be this informative wiki-topic you are after, then? http://www.creationkit.com/Multibounds

They're a "box" that is used to help the game engine decide what to render (like the rooms and portals, but on a smaller scale)

Maybe this single-line explains it best? http://www.creationkit.com/Reference#Multibound

Unless I am mistaken, the word multibounds is a collective term for all of the different types of "bounds" (and/or just a fuller name for "bounds")


http://www.creationkit.com/ToggleMultiboundCheck - You can see what the things do, by turning them off

The http://geck.gamesas.com/index.php/Occlusion_Culling article for the GECK is still relevant.

Basically if you set up room bounds for an interior, you'll generally notice times where sometimes an object will become invisible when your facing certain angles or get far enough away or something like that, so by forcing a multibound for the object that is glitching out, you can force it to render while the roombound you link it to is being rendered.