It's a formula used to solve equations of the form ax^2 + bx + c = 0 where a, b, and c are known coefficients and x is the unknown variable. Here is a link with the formula and derivation:

You can program the formula into a graphing calculator or use an online quadratic equation solver. If the graph of the parabola y = ax^2 + bx + c does not intersect the x-axis, the two roots will be complex, otherwise they will be real.