Typically, a Reddit AMA is live as people throw questions at the subject, and the person tries to keep up with the barrage by responding as best as possible. But for Hawking's AMA, you'll present the questions ahead of time, and he'll answer them over the following weeks.
Specifically, you can submit your questions starting Monday July 27 at 8am ET through Tuesday, August 4, at reddit.com/r/science, and Hawking will respond over the next few weeks. Hawking, who suffers from amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig's disease, uses a computer to communicate. So the delayed format will allow him the time necessary to answer questions."