Velocity is not a Scrum term, it is a term used in Extreme Programming.
Velocity is the rate at which value has been delivered.
It's calculated as an average of the value delivered by a team in the past few Sprints. [ Let's say average of last 4 or 5 Sprints ].
Let's say: The Team Acers delivered below number of Product Backlog Items:
Sprint Number of Product Backlog Items Delivered 5 7 6 6 7 7 8 5 9 8 10 7
At the end of Sprint 10, the velocity of team Acers is 6-7 Product Backlog Items per Sprint.
The velocity trending upward over a period may indicate that the team is delivering more output.
More velocity does not equal more value or more impact.
Also, velocity numbers can easily be gamed when velocity is used for micro-managing the team or pushing them to deliver more than what they could comfortably deliver.
Comparing Velocity of two teams is usually a waste of effort and futile. As two teams may have different set of work, complexity, skill set, collaboration, availability of Product Owner and many other variables would be different.