This menu item links the selected object to the current IK handle by adding the read-only attribute Ik Blend to the object. See Ik Blend.
Once the object is connected, you can select the object and use Animate > IK/FK Keys > Set IK/FK Key to set keys on the IK handle and all the joints in its joint chain. See Animate > IK/FK Keys > Set IK/FK Key.
You can also constrain or parent the IK handle to the object if you want to use the object to manipulate the IK handle. Objects that control and drive the transformations of IK handles are called control objects.
If you turn on Display > Heads Up Display > Animation Details when the object is selected, the Ik Blend value for the IK handle appears in the scene view.