we know that
The vertical line test is a method that is used to determine whether a given relation is a function or not
A function can only have one output, y, for each unique input, x
If a vertical line intersects a curve more than once then for one value of x the curve has more than one value of y, and so, the curve does not represent a function
see the figure below to better understand
In the figure above f(x) represents a function, because the vertical line intersects the graph
