This math video tutorial explains what a function is. It is a special relation where there is exactly one output for every input. This video explains how to evaluate a function and how to determine if a graph represents a function using the vertical line test.

Functions and Graphs:

https://www.youtube.com/watch?v=miTyZbwWCVU

Composite Functions:

https://www.youtube.com/watch?v=ZFPkQkURSxk

Evaluating Functions:

https://www.youtube.com/watch?v=HyNie_PYgsY