In history-dependent inverse kinematics (HD IK), the end effector is the pivot point of the selected child object at the end of a kinematic chain.
The kinematic chain is a single branch of a hierarchy used for animation with inverse kinematics (IK). The chain starts with the selected child object and travels up through ancestors until it reaches the start of the chain. When you move the end effector, the HD IK solver then uses IK calculations to move and rotate all other objects in the kinematic chain to react to the object you moved.
The end effector has two transforms: one that connects it to its parent, and another that connects it to the End Effector Parent. By default, the End Effector Parent is none (equivalent to World); you can assign this in the Motion panel. See End Effectors group.