Negative total float effectively means that the schedule cannot be accomplished as planned. Another way of looking at it is that the project will take X (time units) longer than was originally planned.
Example: Total float = -7 days is the same thing as saying the project will take an additional 7 days to finish.
Training for Project Management Professional (PMP)®, PMI Agile Certified Practitioner (PMI-ACP)®, and Certified Associate in Project Management (CAPM)®