While clearly a marketing term it's used for the "best" portion of the fish, most notably tuna loins (where it's been used for a long time to represent the portion analogous to beef tenderloins). So in packaging it has no real meaning but with a good fishmonger it's useful shorthand.
The "loin" of a fish is a solid piece of meat not unlike a beef tenderloin. You'll sometimes find it offered from large fish species - Monkfish, Swordfish, Tuna, etc. "Loins" are sometimes offered whole for sale, but normally are presented at the fish market so that "steaks" can be custom cut from them.
"Filets" are cut/sliced horizontally from fish, along the bone (to render the filets boneless). Fish like tuna, swordfish, marlon, shark, etc. are cut into loins & steaks because their bone structure is different from the normaly filleted other fish species - sole, flounder, salmon, etc., etc.
But the above rules aren't set in stone. Do some research re: fish cuts to learn what cuts are available & what might work best for what you want to cook.