Images generated by GAN (Generative adversarial network) on thispersondoesnotexist.com show faces of non-existing people - they are computer-generated. But the algorithm always puts the eyes in almost the same position in the photo, as well as the mouth. By overlaying the pattern on these photos, the observer can determine if the face was GAN-generated and doesn't belong to a real person.
You can also look for non-matching earings, glasses end pieces and background deviations.
Please keep in mind that it is only a simple tool in its initial version and it can be wrong.
Do not rely absolutely on the results from that tool. A thorough analysis is key.