total float
The Total Float calculation answers the question, "If Activity X is delayed, how long with the project be delayed?" if an end date is provided to the schedule, the Total Float value can tell you if you are ahead-of-schedule, on-schedule, or behind-schedule.
One typical misunderstanding about float is that float belongs to individual activities. It does not. Float belongs to a path of activities. The path of activities with the least float is called the "Critical Path." This is because any delay to any activity in that path of activities will delay the entire project by that same amount.
Formal notation and calculation method for the Total Float Calculation is covered fully in the sections related to Arrow Diagramming and Precedence Diagramming.