The equation represents the discriminant of a quadratic. It is the part taken from under the radical in the quadratic formula.
For any quadratic:
If the discriminant is positive, or greater than 0, the quadratic has two solutions
If the discriminant is equal to 0, the quadratic has one distinct real solution (the solution is repeated).
If the discriminant is negative, or less than 0, the quadratic has zero solutions
In the graph, we see that the equation intersects the x-axis at two distinct points. Therefore, the quadratic has two solutions and the discriminant must be positive. Thus, we have .