Inverse kinematics (IK) uses a goal-directed method where the animator positions a child object and 3ds Max calculates the position and orientation of the parent objects. The final position of the hierarchy after all of the calculations have been solved is referred to as the IK solution.
Applied IK requires that one or more parts of your IK structure be pinned to animated follow objects. Once pinned, you select any object in your kinematic chain and click the Apply IK button. 3ds Max then calculates the IK solution for each frame of the animation and places transform keys for every object in the IK chain.