This optical illusion features a grid of black and white squares that appear to shimmer and sparkle when viewed from a distance. The illusion is created by the interaction of light and dark squares, and the effect is enhanced when the viewer moves their eyes around the grid. The Scintillating Grid
From Wikipedia
A grid illusion is any kind of grid that deceives a person's vision. The two most common types of grid illusions are the Hermann grid illusion and the scintillating grid illusion.