You're driving along at one speed.  Let's say its 15m/s.
Then just a few seconds later, you're going 30m/s.  What has happened?

Of course you sped up.  In physics lingo, you accelerated.  If you speed up acceleration is positive,  and when you slow down it is negative.

We have spent time learning about speed and velocity. How would you describe your motion while you are accelerating?  Mi/Hr won't work because as soon as you say the number, the speed is different. You could try to look down at the speedometer every single instant and report the number you see.  "16! no wait!  18! uh, no 22!"
This is inadequate.

What we need is a way do describe "how fast how fast changes."
To get what we need, think of what we have....

We have a speed that changes over time.

This is enough.  acceleration=speed change over time.
Mathematically, this is a=Vf-Vo/t=DV/t

What units would acceleration be in?  It only depends on what speed and time units you use.
If your initial and final speeds are given in m/s and the time change is in seconds, then the unit for acceleration will be m/s/s.  Some people say this as m/s².
Login to Lecture OnLine and answer the question called, "acceleration response".

*Acceleration is not the same as speed!  Acceleration is how fast speed changes.
*Acceleration does not always mean you are going faster.  Slowing down is also acceleration, just negative.
*Acceleration does not have to be in the direction an object is moving.  If a car hits the brakes, the acceleration is negative.  This means the acceleration is in the opposite direction of the car's motion.