How to define and use a function
Passing parameters to a function
Pre-defined functions that are available to you "for free"
The concept that functions are just data, albeit a special type of data
Understanding these topics will provide a solid base that will allow you to dive into the second part of the chapter, which shows some interesting applications of functions:
Using anonymous functions
Inner functions (functions defined inside functions)
Functions that return functions
Functions that redefine themselves...