Imagine you have a ring and you want to fill as much of this ring as possible with one type of shape (triangle, square, hexagon, must be the entire shape repeated within the ring no overlaps). To be efficient filling up the ring you want the total number of shapes to be as low as possible. So which shape will have the best ratio of area filled up per number of shapes used (A/n). linked are some simple examples using squares (area of the circle = 1).
Edit: n>1, only straight edge shapes Edit 2: shape must tile the plane