An external reference is created when one document is dependent on another document for its solution. If the referenced document changes, the dependent document changes also.
In an assembly, you can create an in-context feature on one component that references the geometry of another component. This in-context feature has an external reference to the other component. If you change the geometry on the referenced component, the associated in-context feature changes accordingly.
To list the external references on a part or feature, right-click the part or the feature or right-click the update holder 
, and select List External Refs.