One of the most important concepts in this course is that of function: I will expect you to be familiar with the major classes of functions (e.g. polynomials, trigonometric, etc.). We'll also meet other classes (e.g. exponential and logarithmic functions).
The most important function is calculus is the linear function (or, more generally, the affine function: y=mx+b).
These are used everywhere in science, business, etc., because they're so useful and can do so much (in particular, approximate more complicated functions over a small interval).
An important example arose while I was up in Canada recently: how do we convert between Fahrenheit and Celsius?
As soon as we answer that question, we'll probably want to know: how do we convert between Celsius and Fahrenheit? That's the inverse function!
Next time: 6.1