Backer Rod is a foam cylindrical gap sealer. You purchase it in lengths. then push it into the gap between the coping and deck of the pool. Leave about 1/4 or 3/16 of an inch from the top to fill with your caulking. You can use Deck-O-seal it is self leveling. I used that. When I put the deck-o-seal in I then used a matching colored sand and piled it on top of the WET deck-o-seal. This leaves a natural looking finish. Let is set for 24 hours then rinse off the sand looks great.
Rick