The velocity of a team is a measure of how much work that the team can handle within a specific time period, i.e. how much of the product backlog can be completed by the team in a sprint. Velocity can be calculated on the basis of story points, business value, hours, issue count, or any numeric field of your choice (see Configuring Estimation and Tracking).
Screenshot: Velocity Chart (click to enlarge)
The velocity can be estimated as the average, over several recent sprints, of the sum of the estimates for the amount of work completed by a team per sprint — so in the chart above, the velocity = (37 + 47 + 50 +57) / 4 = 48. A team's recent velocity can be useful in helping to predict how much work can be completed by the team in a future sprint.