I did access to business a few years ago to go to uni for an economics degree. This is how it worked for me but check with your course provider to make sure it's the same.
The course is made up of 60 credits, 15 at level 2 and 45 at level 3. Level 2 is normally considered GCSE level and level 3 a level.
The level 2 units are pass or fail and so therefore are not graded. Level 3 units are graded fail, pass, merit and distinction.
Each unit will make up a number of credits, for me it was either 3 or 6 credits per unit. So for entry to the degree course you will need to pass all level 2 units and get level 3 units totalling 30 credits at distinction and the rest of your level 3 units at merit.
In my course the level 2 units were very basic and included units on study skills (planning study time and self reflection).
The level 3 units could be essay or exam based. I only did 2 exams, each worth 6 credits. The essays I did were a mix of producing leaflets that could be used to explain something to someone with no prior knowledge, full projects (mini dissertation of around 5,000 words but split over 2 6 credit units) and straight essays.
The offer you have is normal for a good quality course/uni. If the offer was lower then some would see it as a signal of the standard of that uni/course. A bit like one uni asking for AAB and another asking for BBC. If you look at the standard uni offers for your course for A levels you will probably find it's around AAB/ABB which is the equivalent of the offer of your access course.
I hope this was too basic as you did ask for a dummy explanation.
Overall do not be too worried your offer is completely doable. I managed to get 45 credits at distinction and I hadn't studied for over 15 years.
If you need anything else feel free to pm me.
Good luck