I wrote a rather lengthy explanation of Metric Reserve which will make the most sense to those who understand Ripple. Metric Reserve really is, in my mind, a derivative of Ripple concepts with different semantics and some extra algos bolted on.
I've made an introduction video to the explanation document:
Here is the link to the explanation. Plenty of diagrams to help understand:
Hope this helps! Most documentation I've written on Metric.
David