If you are asking about the opening at the spindle end of the crank arm (the visible female threads at the extreme left edge of your photo) you need a crank arm dust cap. These vary a bit in diameter and threading among crank manufacturers but the Shimano type is most common and any bike shop should have them at low to no cost. They are available in metal or plastic and are installed and removed with a large screwdriver slot, a hex key hole or two small holes for a pin spanner depending on make.