You should have got a very detailed email with your tickets to the show. It clearly explains what will happen. The 'dry run' on the Friday (ignore the jolly japes above!) is basically a full dress rehearsal, complete with TV cameras, which will run exactly as the first show will run 2 days later. The email tells us what time to rock up, where to park, dress code, etc. etc.
Although there is nothing to indicate this at the moment, I know from other TV recordings that it could be that they actually record musical guests or group routines on that day, along with audience applause or other items that could be slotted into the live shows. (This is why we are asked to follow the dress code.) Also, if there is some sort of major disaster the show could be broadcast in place of the first live show, though I don't think this has ever happened - I'm sure someone will correct me if I'm wrong! The chances are 99% we will not appear on the telly. The show we watch on Sunday 7th will be recorded live with a new audience but should be very similar (barring falling over of course!) to the one we see on the Friday.
Hope that helps. I know as much as you do. Check the email, it is very detailed.