The discriminant of a polynomial of the form , is . The discriminant is an important feature which, among other things, helps us to determine the nature of the roots of the polynomial.
In your case, you have , so the discriminant will be .
Recall that the solutions to the quadratic equation are, by the quadratic formula, . Whether there are two real roots, one double root, or two complex roots depends upon whether the number in the square root, is positive, zero, or negative.
That number, is the "discriminant".
As row said, in your case, the discriminant is -71. That tells us that the equation has two complex roots.
It als tells us that the graph of does not cross the x-axis but is entirely in the upper half-plane.