The discriminant factor is a correlation between students that answer this question correctly and how well those students performed on the test overall. Expressed as a decimal between -1 and +1, a positive discriminant factor indicates that students that did well on this question did well on the overall test. A strong negative value may indicate a poorly written question or incorrectly keyed answer.
Generally speaking, well written exams with enough questions and students to be statistically significant try to have discriminant factors over 0.2 with higher being better.
ZipGrade calculates the discriminant factor as a Pearson's correlation: http://en.wikipedia.org/wiki/Pearson_product-moment_correlation_coefficient
Here is a great teacher write-up on using discriminant factor to better understand your quizzes: