The External Link tool is a simple tool that redirects to the configured URL. This is often used to provide a navigation bar item that points to external code repositories, forums, or donation systems.
The External Link tool is installed via the "Add New..." link in the project menu bar, just like any other tool.
The Label will determine how the link is displayed in the navigation bar.
The Url Path will determine the URL of the tool before it redirects, and is not too important.
The External Link field specifies where this link will go to.