Prefabs are a predefined group of placeables positioned in a coherent manner.
Once you place a prefab on a map, all placeables from the prefab are grouped together and can be moved / rotated together.
Save it under Program Files/Atari/Neverwinter Nights 2/NWN2Toolset/Prefabs (should be the default folder).
.pfb file from Program Files/Atari/Neverwinter Nights 2/NWN2Toolset/Prefabs to Document/Neverwinter Nights 2/Override, otherwise the prefab won't appear the next time you open the toolset..git in order to open / modify it. Don't forget to change back to .pfb when you're done.