At it's core, think of Compass as an ORM tool, but one that works with a Search Engine (and provides full text search) instead of a databases. It gives you the ability to perform full text search on your domain model. Think for example of a portal application, and one of it's portlet displays email items, and the other displays news items. With Compass, you can provide a "google like" search box on your emails, news, or both of them, without actually worrying about low level search engine stuff. The results of the search will be your News and Email objects (which you can format however you want).
Once we understand the OSEM - Object to Search Engine Mapping technology, than the next obvious step is to try and integrate it with other Object mapping frameworks, like ORM tools. Compass provides integration with several ORM tools, like Hibernate, and can automatically synchronize changes made to objects by Hibernate that have both ORM and OSEM definitions. This feature helps integrate search engine capabilities to an ORM enabled application a snap.
Hope it is a bit clearer, if not, head over to the site and read the About
sections. They go into more details explaining this stuff. And of course, there is always the documentation