This is a classic illusion named after Ludimar Hermann who discovered it in At every point where the white lines intersect our eyes perceive a gray, shadowy blob. If you look directly at one of the intersections though, the blob disappears.
Illusions are always mind freaking. An optical illusion (also called a visual illusion) is characterized by visually perceived images that differ from objective reality. The information gathered by the eye is processed in the brain