Negative float is the amount of time by which the early date (start or finish) of an activity exceeds its late date. Learn more about it here:
Compare this with Total Float and Free Float on a Network Diagram.
Total Float is the amount of time that an activity can be delayed from its early start date without delaying the project finish date. Free Float is the amount of time that an activity can be delayed without delaying the early start date of any successor activity.
The following article explains the Float concepts in detail:
What is the difference between Total Float and Free Float?