The Amsler grid is an early warning system for a change in your parent's macular degeneration. Macular degeneration affects the retina of the eye, which is sensitive to horizontal and
vertical lines. If you look at the grid that your parent has posted on the refrigerator, you'll see that it has evenly spaced horizontal and vertical lines, with a dot in the center.
To use it, your parent will stand 18 inches from the grid (with her reading glasses on, if she wears them) and look at the center of it. Using one eye at a time, she'll note any distortion in the lines of the grid. The lines may look wavy instead of straight, for example, or she may not be able to see parts of the grid (for example, there may be holes, or she may not be able to see the grid's corners).
Changes -- which she should report to her eye doctor immediately -- may signify that her condition has progressed from dry to wet macular degeneration, or that her wet macular degeneration has changed.