There are effects you want to place before an amp (either in POD or with a real amp), and effects you want to use in the effects loop of a real amp - that is, between a preamp and a power amp section. In POD it would be after an amp. FX loop of an amp is like a hole in the chain between pre and PA where you can plug whatever you want.
If you introduce FX loop block (a hole) in a POD chain, you split the chain in two parts so you can add external effects to that hole, or route parts of the chain before a real amp and in the FX loop (a hole) of the real amp.
Kopfschmerzen's post in FX Loops help was marked as the answer