This is a common problem which can be worse if the countertop is out of square. It happens because the walls expand and contract and cause the gap to get wide or shrink dependent on the weather. There are two ways to address this. If the gap is less than about 1/4 inch or so, you can simply caulk the crack. I'd use acrylic latex caulk for that. It is flexible and will stretch as the walls move with the weather.Your second option is to add a small piece of molding across the top of the countertop splash edge to cover the gap. If you do this, be sure to only attach the molding to the top so that it will "ride" back and forth as the top moves and keep the crack covered.