How long an activity can slip before it delays the activity that follows it. Measured against the next task, not against the project finish.
Free float is the slack an activity has before it pushes its immediate successor. Total float is the slack it has before it pushes the project finish. An activity can hold days of total float and none at all of free float, which is the case that catches people out: the task looks comfortable on the plan, and the moment it moves, the next one moves with it.
The practical use is in sequencing. Total float tells you whether a slip matters to the delivery date. Free float tells you whether a slip is about to land on a colleague this week. Those are different conversations, and a schedule that only reports one of them is answering half the question.