A resource is a customizable unit that can be broadly classified into two types dynamic and static. A dynamic resource is a resource that resolves to application code and executes dynamically on the server. For example, when you log into Parallels Pro Control Panel and click on the option List Sites, the action executes application code to obtain the required result.
A static resource is a resource that resolves to a static file on the server, for example, an image displayed in Parallels Pro Control Panel is a static resource as it resolves to a static image file on the server. This document explains static resource customization.