negative float

Basically negative float is the amount of time the project is behind, as determined by the total time for the tasks on the critical path exceeding the time available for the project.  There is only zero or negative float on the critical path; by definition the critical path doesn't have positive float.  Negative float is something you want to fix.

http://www.projectmanagementquestions.com/2538/when-to-use-a-negative-float