Year 12

I’m starting year 12 in September and I have no idea what AS exams and I do bio chem and maths

In summer of year 13, you will sit A Level exams, which tests all year 12 and 13 knowledge.
In summer of year 12, your school with either make you sit their mock tests or do AS exam (in my sixth form, only students studying 4 A Level had the sit AS exam and if they did badly, they had to drop 1).
Most students do not sit AS exams. If your school does offer it, it makes good exam experience.