It gives you access to some online resources on the topic. I do believe CELS staff have, in 2011, set some quizzes there, but I didn't do any, and IMO it was not necessary.
The CELS textbook is not one that matches the contents of the course very well in the first place, so you'll find that some of the readings don't necessarily align with the Lecture Objectives (to be provided to you in lecture handouts, don't worry, you'll get these at the first lecture, which is a housekeeping one). By that I mean as marsupial said, it's not all relevant (actually, pretty much half the book is irrelevant, and the relevant parts often go deeper than CELS needs us to go), so the access kit isn't all that relevant, either.
Yes, there are some interesting animations (which some lecturers like to play in the lecture) but it's a supplementary thing, rather than a necessity. And yes, the CELS staff have put up quizzes on the Mastering Biology place, but again, they're more as a supplement than something you're expected to go and do - it's certainly not worth any marks.