It is used to see if a graph represents a function because if there is 2 dots or intersecting lines within the other lines then it is not a function. if there is only one line or dot, then yes, it is a function. hope i explained it well.
The graph of a function must pass the vertical line test. When a vertical line passes through the graph of a function, it will intersect the graph at only one point. More than one point on any given vertical line means that the same x is being paired with two different y's, and, therefore, the graph is not a function.
