So if I get a B in science in the year 10 mock exam, my predicted grade would be an A, right? If yes, does it apply for all the subjects.
It's literally what the teacher thinks you'll get if you carry on with the way you're working - they'll look at your progress throughout the year. If you just about got a B, they may predict you a B. However, if you were close to an A or you've been doing well in class they can then predict you a higher grade.