The document discusses functions and relations. It defines a relation as a rule that relates inputs from one set to outputs in another set, while a function relates each input to only one output. It provides examples of relations and functions. The key difference between relations and functions discussed is that a function pairs each input with only one output, while a relation can pair an input to multiple outputs. The document also discusses using functions to model real-life situations and how to represent functions graphically using a vertical line test.