Slow in and slow out

This refers to the spacing of the inbetween frames at maximum positions. It is the second and third order continuity of motion of the object. Rather than having a uniform velocity for an object, it is more appealing, and sometimes more realistic, to have the velocity vary at the extremes. An example of this could be a bouncing ball, as it moves faster when it approaches or leaves the ground - and it goes slower as it approaches/leaves its maximum position. The name comes from having the object or character "slow out" of one pose and "slow in" to the next pose.