I think they mean when you decide not to carry on an A-level, you cash in your AS grade for that subject.
'Cash in' just means that you've accepted you are going no further with that qualification and you want it to be certificated.
So say for me, I'm taking Maths, Physics, Further Maths, Biology and Chemistry.
At the start of next year, I'm going to 'cash in' my biology and chemistry - meaning I have an AS Level in Biology/Chemistry and that's what will go on the UCAS form for those subjects.
I'm going to carry on with Maths, Further Maths and Physics and so won't be cashing them in (will cash in the full A-Levels once I've finished Year 13) - so for these subjects, UCAS will want my PREDICTED grades for the entire A-Level.
Make sense?