Speed is how fast an object is moving. Speed is calculated by dividing the distance traveled by the time of the motion. Velocity is speed in a certain direction. You can run every which way but not get anywhere, unless you run in one direction. If you run with velocity you run in a certain direction.
Acceleration is the change in speed during a certain period of time.
If you put your foot on the accelerator, you will change your speed. That's positive acceleration. If you remove your foot from the accelerator, you will slow down. That's negative acceleration . Positive acceleration is speeding up or changing speed from slow to fast during a certain period of time.