Inverse kinematics calculates the position and orientation of objects in a kinematic chain.
The kinematic chain is defined as a single branch of the hierarchy that starts with a selected child object and continues up through its ancestors until it reaches the base of the chain. The base of the chain is either the root of the entire hierarchy or an object that you specify as a terminator for the chain.
3ds Max automatically determines the kinematic chain when you select and transform an object with the IK button turned on.