When Lock Tangents is inactive, dragging a tangent handle affects only the single key displaying the handle. When Lock Tangents is active, dragging a tangent handle affects the handles of all selected keys.
Lock Tangents: Keeps tangents oriented relative to one another after the SHIFT key has been used to manipulate them.
Inactive: dragging a tangent handle affects only the single key displaying the handle.
Active: Dragging a tangent handle affects both handles of all selected keys.