What specifically do you need help understanding? There are plenty of resources that discuss the architecture and your best bet is probably at http://docs.orchardproject.net/. Orchard's source can be overwhelming
at first but if you start by learning specific modules and how the pieces fit together it starts to make sense.
I guess I know what inbreaks says since I am just meeting the same problem.
I am planning to do something for contributing the Orchard project, as this is really a sophisticated organized and powerful project.
My co-workers and I have read articles and documents on Orchard for days, but we get some troubles on understanding some parts of it.
So we're so hungry for any available resource. Each real document item(code comments, UML, physical data model, entity model diagrams, etc.) can strongly help us inprove our efficiency.
Thanks a lot for reading.