what is the meaning of velocity?

Velocity is the displacement of an object over the amount of time.  It is not the same as speed.  If an object travels 4 miles forward and then 4 miles back all in one hour, its speed is 8 mph. However, displacement is the difference between the final position and the initial position. So in this case, the average velocity would be 0 because its final and initial positions were the same.