In mathematics, the vertical line test is a visual way to
determine if a relation represents a function.
For a function, each x-coordinate
corresponds to exactly 1 y-coordinate.
The vertical line test tells us that if each x-coordinate on the graph corresponds to exactly 1 y-coordinate, then any vertical line that we draw on the graph should hit the graph at only one point.
I have attached an example below.
Notice that any vertical line we draw hits the graph at only one point.