Schedule float (also called slack) is the duration a task can be delayed beyond its earliest start date without delaying the project completion date. Tasks on the critical path have zero float by definition; any delay to them delays the entire project. Tasks off the critical path have positive float, sometimes a few hours, sometimes weeks.
Float is calculated automatically by scheduling software once tasks and dependencies are entered. Project managers focus their attention on critical-path tasks (zero float) because these are the bottlenecks. Tasks with significant float are flexible and can absorb minor delays without project-level consequence. Misunderstanding float leads to misallocated attention.