Ayende has published an excellent sceencast introduction to Rhino Mocks. If you are at least somewhat familiar with Test Driven Development (TDD), then I highly recommend that you watch it.
Rhino Mocks is a mock object framework for TDD, and in case you don't know (I'll assume nothing), mock objects are a tool that enables you to test just one thing at time. In other words, it helps keep the unit your unit tests.
Additionally, Ayende is a magician with ReSharper. I learned a lot about my under-utilization of ReSharper and I was already an enthusiast!
He does move fast and makes a couple of comments that might pass over your head if you don't know a little bit about mocking already (e.g., the explicit record and play model vs. implicit; something he talks about here.)