Students will learn that functions can be defined to take more than one input variable. Students will learn that the domain of a multivariable function consists of tuples. For example, a function with two inputs has a domain consisting of ordered pairs. A function with three inputs has a domain consisting of ordered triples, and so on. Students will also learn that a function with two inputs can be graphed as a surface in 3D space. The first axis is the first input of the function, the second axis the second input, and the third axis the output of the function. Here's a good video on all that.