A prefab is an object standing in space, with fitted
textures but without the entities which makes a map out of it (like
lights and startpositions). So you can load it in your map and then
bring it in the right position to make your map more detailed.
2.
How to use prefabs
First you have to tell qeradiant where your prefabs
are, to do this click EDIT and then PREFERENCES
There you must enter the location of your prefabs
click on image to enlarge
But before you load your prefabs for the first time, make sure that
you have activate MOVE TEXTURE LOCK and ROTATION TEXTURE
LOCK.
After all these you can load your first prefab, to do this go to EDIT
and then click LOAD PREFAB.
Now you only have to bring your prefab in the right position and then
you can deselect it.
If you need some cool prefabs go to the prefab
site from gamedesign.net.