Velocity measures the average number of story points a team completes per Sprint. It is a diagnostic tool for Sprint planning and release forecasting, not a performance measure. The moment Velocity is used as a target or a comparison metric between teams, teams begin inflating their estimates and the metric loses all usefulness.
In practice, Velocity is averaged over several Sprints. If a team consistently completes around 25 story points per Sprint and the backlog still contains 100 points, this yields a rough forecast of about four Sprints to completion. This calculation is deliberately approximate but provides an empirically grounded basis for conversations with stakeholders. During Sprint Planning, the team uses its own Velocity to estimate how many items it can realistically take into the next Sprint, rather than overloading itself.
Velocity originates from Extreme Programming and was introduced by Kent Beck. As an alternative, Throughput — the number of completed items without estimation — is gaining traction, especially in Kanban-oriented ways of working.