No easing
http://www.swfup.com/file/159753
Good easing
http://www.swfup.com/file/159750
See how the ball accellerates and gets slower at various points in it's path? That's easing. For application in an actual animation, take a look at the stick figure kicking. His leg starts out slow, then speeds up for the kick, and when it reaches it's peak, slows down and stops. Then it comes back down, accelerating until it touches the ground. Easing is adding more frames at points in your movements to make it look more realistic. Things don't move exactly the same difference every frame, it will usually change a bit.
Hope this clears things up. If not, just tell me which part needs clarification still