The purpose of the Amsler Grid is to identify distortion in your visual field.
To accomplish it, sit approximately 18 inches from your monitor screen and close either eye. Stare at the center of the grid, and observe the surrounding area peripherally. If any lines are distorted ("bent"), this is evidence of swelling of your retina, and you should make an appointment with your vision doctor as soon as possible.
If you have already been diagnosed with retinal degeneration, continue to use this grid to monitor any changes.