The pivot point defines the position around which objects are rotated or scaled. By default, the pivot point of an object is located at its center. If you want to rotate an object around a specific point, like a forearm rotates from an elbow, you need to adjust the pivot's position. Custom pivot editing mode is the default method for adjusting object and component pivot points.
Custom pivot editing mode (Hotkey: Insert or d) lets you set pivot position and axis orientation. You can use this mode to reposition the pivot point and perform more complex edits like pivot pinning and orienting and snapping the custom pivot to a component.
For more information on specific custom pivot editing workflows, see Orient the custom pivot to a component and Reset the custom pivot.