Vertical line test helps to determine whether or not a graph represents a function. If a vertical line touches the graph at more than one point, then the graph is not a function. On the other hand, if the vertical line is moved across the graph and, at any time, touched the graph at one point, then graph is a function but that isn’t the case. Moreover, since this graph wasn’t a function, the vertical line touched the graph at more than one point.
Ask yourself if it is possible to draw a single straight vertical line through more than one point on the red parabola. Since such as task is not possible in this case, we consider this curve to be "passing the vertical line test". It means that any x value input leads to exactly one and only one y value output.
