A girl who is normally considered ugly, but is now considered hot because of the fact that you’re in an environment where few women are present (such as the desert or boot camp). The 10 part refers to the hotness scale (10 being the hottest, 1 being the ugliest).
All of the girls that are normally 1’s and 2’s become 9’s and 10’s in the desert.
Better soak it up while you can desert 10, because once we get to the states, you’re back to 2 status.