Slo-in and Slo-out, also called ease-in and ease-out by the computer animation crowd, is a fundamental concept in animation. The idea behind it is to give the illusion of an object accelerating and deccelerating. For example, if you were to watch a car sitting at a stop light, then the light turns green and the car begins to move up to a speed of 30 mph, the car doesn't go from 0 mph to 30 mph in an instant. It takes time to get up to speed. 0 mph, 1 mph, 3, 6, 10, 15, 23, 27, 30.
To achieve this illustion in animation, you must be mindful of spacing and timing. The space the object moves on screen would become greater and greater the more time elapses. That is to show an object accelerating. To show it deccelerating, just do the opposite. The object would move through space at ever decreasing intervals.
Lesson 2A
Lesson 2B
Lesson 2C