Point-Biserial is a process of looking at the students with the highest scores on the test and the students with the lowest scores on the test and comparing them against each other for a particular question.
If the Point-Biserial is a positive number (has a value between .01 and 1.00) it indicates that the students who scored in the top 50% of the class on this test answered the questions correctly more often than the students who scored in the bottom 50% of the class on the test.
This would be the expected behavior, as students who performed best on the test were likelier to answer that item correctly.
If the Point-Biserial is 0.00 it indicates that all students performed the same way on this question (either all students answered it correctly or all students answered it incorrectly).
If the Point-Biserial is a negative number (has a value between -.01 and -1.00) it indicates that students who scored in the bottom 50% of the class on the test answered the question correctly more often than the students who scored in the top 50% of the class on the test.
This would be unexpected behavior. We see this often with questions where students who studied hard were taken in by an attractive distractor, while students who didn’t study as hard may have just guessed correctly. This generally indicates that the question may warrant review to determine why the lowest performers on the test were getting this question correct more often than the highest performers.