The vertical line test is simply using a vertical line across the whole graph to see if it touches two points at the same time. This is used because a function can only create ONE output for every input. For example, you can't plug 2 into the function
and get two different answers, you'll only get one, 4. However, you can also plug in -2 to get 4. This is because two different inputs can have the same output, but one input CANNOT have two outputs.
Therefore, if it fails the vertical line test, then it's not a function