This is an Amsler Grid. It is used by eye doctors to help determine whether there is damage to the macula of the eye.
You can use it at home by following these steps:
Cover one eye and look at the dot in the center of the grid.
Notice whether any of the lines look wavy or distorted.
Notice whether any parts of the grid seem to be missing.
Repeat with other eye.
If you notice any distortion or parts that are missing, it is a good idea to contact your eye doctor for an exam. Many problems with the macula can be repaired more easily if they are detected early.
If you are having a problem, don’t wait. One reason that I can see so well today is that I went to my eye doctor right away when I noticed blurring.