This is a variation of the Hermann Grid where black dots appear and disappear at the intersections of the gray lines. Interestingly enough, if you cock your head at a 45 degree angle the effect is reduced (but not eliminated).
ART TEACHER: HOLLY BRIGGS "5th grade optical illusions tiered for all ability levels. Two color, two color challenge pattern, three color challenge pattern, four color and beyond. These are all from one class! Elementary art on point!"