Limb darkening is a visual effect that occurs in stars, or the sun in this case.
The edge, called the limb, of the sun is darker than the center of the sun.
Therefore, it is clear that if the limb is the edge of the sun, limb darkening is when the limb is darker, and has a lower level of brightness that the middle of the sun.
Hope this helps, let me know if you need anything else! :)
