The discriminant tells you where the graph of the parabola goes through the x-axis, if at all. If the discriminant is negative there are no real zeros and the parabola does not cross or touch the x-axis; if the discriminant is positive the parabola will go through the x axis in 2 places; if the discriminant is 0 the parabola will touch the x-axis in 1 place. Our discriminant is 0 since the parabola only touches the x-axis but does not go through.
