Basically, in a function, you draw a vertical line. If that vertical line crosses the function more than once, then it is an invalid function.
Let me know if you need an example and I'll edit a picture in for you.
Answer:
A vertical line test is used to find out whether if a curve is a graph of a function or not. If you are able to draw a vertical line, and it intersects more than one point on the curve, then it is not a function.
Step-by-step explanation:
