A LANSA application consists of many objects, Forms, Files, Fields and so on. These objects are managed in the LANSA Repository.
Often there are 'other objects' that are a part of your application, such as Javascript, HTML-pages and images for web development, or even simple configuration files for desktop applications.
External Resources have been developed to manage these files. They can be registered in the Repository, checked into an IBM i master and deployed using the LANSA Deployment Tool.
External Resources are LANSA objects just like any other, but with the exception that the LANSA Editor doesn't know how to edit them—as it has no idea of its contents—and LANSA programs don't know how to use them—the code will still be just referring to a file on disk. You do, however, want to register them in the Repository. This has two advantages: as a back up and to make it easier to correctly deploy them with your application.
Also see
Register Multiple External Resources
Register Individual External Resources in the Visual LANSA User Guide
External Resource Definitions in the Technical Reference Guide.