Sunned means faded by light. On books it usually affects the spine of a dust jacket or binding, the part that faced out on a shelf, but it can affect whole covers left in a window.
How to spot it
Compare the spine with the front panel of the jacket or binding. A sunned spine is paler, often in a sharp band where the shelf edge or a neighbouring book shaded part of it. Red, orange and some blues and greens fade fastest. Look too at the top edge of the pages and at cloth boards under the jacket, which can fade unevenly.
What it does to value
Fading is permanent, and collectors notice it at once because the spine is what they see first. Light sunning lowers the grade a step; strong fading that has lost the lettering or colour on a key jacket can lower value a good deal, especially for jackets famous for their bright spines.
Keep books out of direct sunlight and away from strong artificial light, and use clear jacket covers. See caring for collectable books and book condition grading.