ModeShape is a JCR implementation that provides access to content stored in many different kinds of systems. A ModeShape repository isn't yet another silo of isolated information, but rather it's a JCR view of the information you already have in your environment: files systems, databases, other repositories, services, applications, etc.
To your applications, ModeShape looks and behaves like a regular JCR repository. Using the standard JCR 2.0 API (a.k.a. JSR-283), applications can search, navigate, version, and listen for changes in the content. But under the covers, ModeShape gets its content by federating multiple back-end systems (like databases, services, other repositories, etc.), allowing those systems to continue "owning" the information while ensuring the unified repository stays up-to-date and in sync.
Of course when you start providing a unified view of all this information, you start recognizing the need to store more information, including metadata about and relationships between the existing content. ModeShape lets you do this, too. And ModeShape even tries to help you discover more about the information you already have, especially the information wrapped up in the kinds of files often found in enterprise systems: service definitions, policy files, images, media, documents, presentations, application components, reusable libraries, configuration files, application installations, databases schemas, management scripts, and so on. As files are loaded into the repository, you can make ModeShape automatically sequence these files to extract from their content meaningful information that can be stored in the repository, where it can then be searched, accessed, and analyzed using the JCR API.
This document goes into detail about how ModeShape works to provide these capabilities. It also talks in detail about many of the parts within ModeShape - what they do, how they work, and how you can extend or customize the behavior. In particular, you'll learn about ModeShape connectors and sequencers, how you can use the implementations included in ModeShape, and how you can write your own to tailor ModeShape for your needs.
So whether you are a developer on the project, or you're trying to learn the intricate details of how ModeShape works, this document hopefully serves a good reference for developers on the project.