Why did you choose the A level subjects you're doing and what do you enjoy about them?
First of all, the admissions tutors are looking to see if you can explain decisions you have made regarding your education and/or career path.
In most cases, at least one of the A levels you have
 chosen will be a prerequisite, or at least relevant, to studying for a 
degree in a particular subject.
For example, if you are applying for a Biochemistry degree, it is likely you are taking A Level Chemistry.
Talk
 about how an A level in the subject will provide a foundation for 
further study at university, and describe a couple of aspects of the 
syllabus that you enjoy.
Consider the other A level subjects you are studying that aren’t necessary or relevant to the degree you have chosen.
For example, as well as Chemistry, you may also be studying A Level Economics.
Why
 did you choose this subject? What do you like about it? And how might 
the skills gained help you with a Biochemistry degree?  
Perhaps you enjoy the challenge of Economic theory, or find the behaviour of firms interesting.
In
 terms of skills, you might say Economics has developed your ability to 
think logically, and improved your analytical skills. These can then be 
applied to areas of Biochemistry to aid your understanding.
So be prepared to justify your decisions regarding A Level subjects, whether they are necessary for your degree or not.