Prefabs is a Volume of blocks meant to represented prefabricated areas within the world. You can save bits of the world, re-load them later, and include them with your mod.
They can also be used to clone sections of the world easily.
In case you want to create/use an external program that takes advantage of prefabs, here is the binary spec for the .pfb format:
|Magic Length||byte||1||The length of the magic header string (val: 6)|
|Magic||char||n||The characters for the magic. Should always be "PREFAB"|
|Version||byte||1||The version number. Currently 0|
|CenterZ||signed int 32||4|
|Blocks||ushort array||2 * width * length * height||The array of blocks. Coordinate determined by: z*width*length + y*width + x|