If you are applying to universities in the UK (via UCAS) or the US, you will submit your application in November or January—months before you sit your actual A Levels in May/June.
So, how do universities know if you are good enough? They use Predicted Grades.
How are they calculated?
Your teachers generate predicted grades based on your performance in the first year (AS Level or Internal Exams), your mock exams, homework, and general trajectory. If you scored a B in your AS mock, your teacher might predict you an A for the final A Level, assuming you will improve.
Why do they matter?
Universities give out 'Conditional Offers' based on these predictions. For example, UCL might offer you a place on the condition that you achieve AAA. If your predicted grades are BBB, they will likely reject you immediately.
How to improve your predictions
Teachers are human. If you underperformed in a mock, but show incredible dedication, ask for extra assignments, and consistently score highly on past papers (which you can print from the RevisionGrid Bundler), you can often politely negotiate with your teacher to bump your prediction up by one grade.