As a heads up, make sure that you study that basic physics and math that you probably haven't had for a couple of years (as already mentioned). When I was there, a HIGHLY qualified candidate got stuck on some of this stuff. Go through the study packet and be prepared to answer any of the questions. Just make sure that you can answer any projectile, conservation of energy/momentum questions (using calc), and calculus questions. The calc question will most likely be on integrating objects up to 3-D in any coordinate system. It is pretty easy to forget a lot of this stuff, but I'm sure that you will do fine.
If you do poorly on the interview, you will either be invited to reinterview, or rejected. A rejection means thanks for playing, try something else.
The officers running the program will be able to give you more details on the process. As mentioned before, they will ask you about your grades, and any other short comings that show up on your application. You are already qualified by virtue of being invited, now you just have to prove it. Nobody gets invited that can't make it in to the program.
Best of luck!