Tenured means that they cannot be fired, even if they are grossly incompetent. Associate professors are a rung below Professors. Sometimes this is because they are at an earlier stage in their career, sometimes because their career has stalled after they were discovered in the chimney of the dean's apartment. These people can be fired, but if they are, it will not be for reasons related to their ability to teach calculus. It does not necessarily mean that they have anywhere to go afterward.
Some colleges hire temporary faculty mainly to teach classes.
This may mean that they actually care about their teaching. In the next chapters, the authors continue to give good advice, including recommendations about doing homework, getting help, studying for exams, and taking exams. The chapter on calculus prerequisites provides a very brief review of some essential ideas. That being said, the explanations here are well done, and while there aren't a lot of examples, they are well chosen. I think my favorite chapter might be Twenty most common exam mistakes and not just because the authors take a cue from The Big Ten athletic conference and include 21 items in their list.