Basically, it takes 7 minutes for EDL or Entry, Descent, and Landing. However, it takes 14 minutes for the information for that landing to get back to earth. So there's a 7 minute period of time where the rover is just in the atmosphere and scientists on earth don't know what's going on until it happens. The 7 Minutes of Terror are the 7 minutes that the rover's in the atmosphere and transmitting info to us where we don't know whether it's going to land okay (technically we don't know if it already landed ok), and we can't do anything to help if there's a problem.