When creating an application based on the WebCenter Portal Application template, you end up with two projects: Portal and PortalWebAssets. You’ll spend most of your time in the Portal project, but you should understand what the PortalWebAssets project is good for. The Understanding the PortalWebAssets Project section of the WebCenter Developer’s Guide gives a good summary:
Static resources for a web application, like HTML and image files, are typically bundled and deployed with the application. Requests for both dynamic and static resources are generally handled by the host application server. One way to optimize server performance is to separate the static resources from the application and deploy them on a different (possibly less expensive and more scalable) server. WebCenter Portal applications support this scenario with a static application resources project, which can include static resources like HTML and images. By default, this project is called PortalWebAssets, but you can rename the project if you wish.