This test can be used to determine whether the given graph is a function or not.
Before going into the vertical line test let's first understand what is a function in a graph? A function can be defined as for every input there can be only one output. In order to test this we need to draw a vertical line on the graph, after drawing the vertical line we have to identify the points where the vertical line intersects the graph, if the vertical line intersects the graph at two points then the graph is not a function, since this graph is having two outputs. If the vertical line intersects the graph at only one point we will be having only one output, then the graph is a function.
This video is very helpful to understand whether the graph is a function or not using vertical line test.