The vertical line test is used to determine whether a given graph represents a function or not. Let's jump into some examples:
Draw a circle. Draw a vertical line through this circle. How many times does the line intersect the circle? Twice. If that's the case, there is more than one y-value for a given x value, meaning that this graph does NOT represent a function.
Draw the line y = 2x -1. Draw a vertical line thru it. How many times does this vertical line intersect the line y = 2x - 1? Once. For each x value, there is exactly one y value. This graph WOULD represent a circle.