This is a bit loose, but should help I think.
Free float with a positive value indicates how late a task can start and not affect the finish date- if a task has a float of 4 days, it could start 4 days late and would not push out the finish date, assuming you have a fixed start and finish date.

A negative float indicates the opposite is true. If you have a critical path activity (float zero) and a critical predecessor finishes late, then it will push out the defined finish date by 3 days. You cannot start an activity 3 days earlier, after all.

If th e-ve float is increasing, it means that you are delaying critical path items. this usually occurs when you update a programme to actual progress, and things have fallen behind.

As an example if you have 3 tasks- A with 3day duration, B with 6 day duration and C with 8 day duration, then the finish date is 17 days after start date. If you have a start and finish milestone with Must start/ finish on, then the time is fixed. If task A takes 5 days, then the actual finish date is pushed out by 2 days. As the finish date is set, you need to gain back 2 days to finish n time. Thus the latest start date for B is -2 days from its programme start date, giving a float of -2, which would also be th efloat on C.

you can also get thisresult if you have intermediate fixed points- say Phase A has to finish on a certain date, it could affect anything before this but not necessarily anything after this.