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. For example, a bouncing ball moves faster as it approaches or leaves the ground and 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.
This is usually achieved by using splines to control the path of an object. The various spline parameters can be adjusted to give the required effect. In 3D Studio this is controlled by the parameters Ease To and Ease From in the Key info window (from the Track info window). When these are zero, there is a constant velocity in either direction, i.e., to/from the keyframe. When Ease To is set to a higher value, the motion is faster as it leaves the previous keyframe and slows as it approaches the current keyframe. When Ease From is set to a higher value the motion is slower leaving the current keyframe and speeds up as it approaches the next keyframe. The tick mark spacing shows the velocity with closer tick marks indicating a slower rate and spaced out ones indicating a faster rate.
There are other potential problems with spline controlled movements. Because of the nature of splines, there may be an overshoot effect, For example, look at box1.flc. The upper box was keyframed to just touch the top of the bottom box, but because of the properties of the spline curve, it overshoots and goes into the lower box. There are three other parameters that control the approach to/from keyframes. These are discussed below.
Reference: Doris Kuchanek and Richard Bartels, "Interpolating Splines with Local Tension, Continuity, and Bias Control", Computer Graphics 18:3, pp. 33-41, July 1984 (SIGGRAPH 84)
Tension controls the overall shape of the spline curve.
Continuity equally affects the angles at which the spline enters and leaves the keyframe.
Bias affects the overshoot or undershoot of the motion through the keyframe.