What is Float (Slack)?
Float (also called slack) is how long an activity can be delayed without pushing the project's finish date. Activities on the critical path have zero float; everything else has some room to breathe.
Float is management information as much as math: it tells you which delays deserve a fire drill and which don't, and where you can safely borrow resources to protect the critical path.
Formula
Total Float = Late Start − Early Start
Worked example
Task C can start as early as day 3 and as late as day 6 without moving the project finish. Its total float is 3 days — a delay of up to 3 days on C costs the project nothing.