The vertical line test determines if a given graph corresponds to a function by checking for multiple vertical intersections. Consider the graph of an ellipse (an oval shape). If a vertical line is drawn through the interior of the ellipse, it crosses the boundary curve at an upper point and a lower point. Because the vertical line intersects the graph at more than one location, a single $x$-value produces multiple $y$-values, signifying that the ellipse does not represent a function. On the other hand, consider the graph of a slanted straight line. Any vertical line drawn on the plane will intersect the slanted line at exactly one point. Since no vertical line intersects the graph more than once, each $x$-value corresponds to exactly one $y$-value, demonstrating that the slanted line represents a function.