This document discusses functions in Processing. It explains that functions can be called or defined, and reviews the structure of a function which includes the return type, function name, and argument list. It demonstrates defining a square() function and discusses the benefits of using functions such as organizing code, modularity, reusability, and readability. An example compares organized code using functions versus disorganized code. The document concludes by assigning reading for the next class and mentioning an upcoming quiz.