If it is behind the refrigerator and it cannot be seen there is no real need to fill it. The gap is actually there to serve a purpose. If the floor is in fact real hardwood the wood is going to expand and contract with humidity and temperature. When the wood expands it need to go somewhere and if there is no gap then what would happen is the floor would buckle.
Floating type floors like Pergo and some engineered flooring also require a gap between the wall and the flooring. This is because the floor floats (moves) so it also needs a place to move about into. when dealing with the latter kinds of flooring there are spacers that should have been placed between the wall and the flooring to keep the flooring from separating at the seams but have a flexibility to them.
The only reason why you might consider filling that gap is because of a possible leak occurring from your fridge. The leak can come from either a water leak from the ice maker or something like the condensation pan under the fridge overflowing. If the gap is not closed up in a manner that is water tight water could makes it way back there into the gap and end up underneath your hardwood floor. if that happens you may never even know that there is a problem until it is too late the the flooring ends up needing to be replaced and possibly end up with damage to anything under that area.
Clear silicone in a tube is what I would use to full the gap. It will not impede the movement of the wood and it will close up the gap and not allow any moisture to get into it. Use Painters tape to tape off the wall and also the floor before putting it in. and once it is in and before it has cured remove the tape. Have a roll of paper towel handy to clean up any mess and make sure to have a garbage can within arms reach to dispose of the tape.
