The document defines what a function is as a relationship between two variables where each value of one variable corresponds to a unique value of the other. It provides examples of functions like supply and price or time and distance. It defines the domain as the set of inputs and range as the set of outputs. The document then discusses operations that can be performed on real-valued functions, including addition, subtraction, multiplication by a scalar, multiplication of two functions, and taking the quotient of two functions.