The vertical line test is used to see if a drawn line on a graph is truly a function or not. A function is defined as for every input (x value) there is ONE output (y value). So if you're able to draw any vertical line through a point and get one y value for every x value, then the graph drawn is a true function. If you drawn a vertical line and you get two y values for one x value, then the graph is not a true function
