What the hey is "sun staining? Good question and one that I found myself asking very recently.
The term was used by a woodworking friend who had built some cabinets for his kitchen several months ago. But he didn't have time to complete them so he ended up installing them without doors which he planned to build later as time allowed. When he finally got around to making the doors, he found that the new wood he used for the doors did not match the older wood he had built the cabinets out of. He was very worried about this and said that the cabinets had become "sun stained."
It took me a little while to realize that what he was trying to say was the older wood had darkened a bit (slight "smacks self in head" moment!).
His "problem" was compounded by the fact that he had used cherry for this project and anyone who has ever worked with cherry knows that it is probably the most susceptible wood in the universe when it comes to "sun staining." I spent the next hour trying to explain that those of us who do this for a living have a word for this phenomenon. We call it "patina" and spend a lot of time trying to convince our customers that this, in fact, is a desirable effect that should be welcomed. Maybe we do this because we know that there is no force or substance known to man that will arrest the color of wood at any given shade.
In the end I managed to get the guy calmed down by assuring him that his new cherry doors would soon match his older kitchen cabinets and that all that would be needed is a little patience while nature did its work.