The document discusses functions in C++. It covers void functions which return no value, call-by-reference parameters which allow a function to modify the argument passed to it, and using preconditions and postconditions to document functions. It provides examples of defining and using void functions, call-by-reference parameters, and specifying preconditions and postconditions for functions. It also gives an example problem of calculating retail price based on wholesale price and days until item sells, which is broken down into subtasks that could each be implemented as a function.