Animation retargeting
Replaying an animation authored for one skeleton on another with different proportions, bone to bone, so a short and a tall character can share the same walk cycle.
Skeletons differ in bone lengths and sometimes in bone count, so an animation bound to one specific rig won't fit another as-is. Retargeting maps the source rig's bones onto the target's — often via a standardised humanoid rig such as Unity's Humanoid or the Mixamo service — transferring angles or positions depending on the mode.
It lets a studio buy a single mocap library and reuse it across the entire cast. Inverse kinematics then cleans up contact points like feet and hands, which would otherwise sink into the ground or float in the air when the proportions don't match.