This is the Hermann grid illusion. It’s a very well-known and simple optical illusion. When you look at the intersections of the white lines, you see illusory dark blobs.
The standard explanation for this has to do with retinal ganglion cells. These cells pool information from several rod and cone cells in the retina and transmit it to the brain. The area each ganglion cell collects information from is called its receptive field. In the middle of each receptive field, light stimulates the ganglion cell, causing it to fire more frequently. However, on the periphery, light has a paradoxical opposite effect: the more light hits the peripheral receptive field, the less the ganglion cell fires. The theory goes that, at the intersections, more of the peripheral receptive field is stimulated than elsewhere, and so the ganglion cell is inhibited, and the area appears darker. Here’s an illustration of this:
This would also explain why, when looking directly at the intersections up close, you don’t see the dark blobs, but they start to appear in the peripheral vision, and as you move back. This is because the receptive fields are much smaller in the fovea, the center of the eye. Here’s an illustration of that:
The conventional explanation is wrong. A recent counterexample shows why:
If you make the lines wavy, the illusion disappears completely. As you can see, this should not, according to the conventional hypothesis, change the illusion:
But it does, and so it appears that there’s something else going on. What, exactly, is still a mystery. The Hermann illusion, discovered almost 150 years ago, isn’t quite ready to let its secret go.