Acceleration refers to the change in the velocity of an object. When you step on the accelerator after a traffic light turns green, the velocity of your car increases; in other words, your car accelerates. Since acceleration is the rate of change of velocity, the units are (meters per second) per second or in more compact form: m/s2.
A common value of acceleration is the acceleration of gravity near the
surface of the earth. The value of this acceleration is a = 9.8 m/s2.
What this means is that if you drop an object from rest, each second its
velocity increases by 9.8 m/s. In other words, after one second
(t = 1 s) its
velocity v = a 
t = (9.8 m/s2) 
1 s = 9.8 m/s.
After two seconds (t = 2 s), v = a 
t =
(9.8 m/s2) 
2 s = 19.6 m/s, and so forth.
Next: Orbital Velocity