As I explained in my Introduction to "Oslo", the "Oslo" modeling platform comes with three main components: the repository, the modeling language (M), and the modeling tool ("Quadrant"). M is probably the hardest of the three to get your head around.
One of the reasons it's hard to get your head around M is because it's multifaceted — it can serve different purposes — and the different use cases aren't immediately obvious when you first jump in. In an attempt to make things clearer, my fellow instructor Jon Flanders has produced a nice introduction to M video that shows how M can be used as a simpler language for generating database tables. Check it out here.
By the way, we now have a central landing page for our screencasts produced by Pluralsight instructors. We intend to continue growing this collection of free screencasts over time.