In these grids, you can see small grey dots bouncing around in the
intersections of the gridlines, except at the grid intersection which
you are actually looking at. The dots appear to move because as you move
your eyes, the one you try to look at disappears, whereas the one you were
looking at reappears.
The reason the dots disappear and reappear is that when you look directly
at something, you point your most sensitive vision in that direction, so that
you gather information with finer resolution. Everything in your peripheral
vision is summed more coarsely, so that at the grid intersections, the
surrounding colours differ to those along the grids.