I subscribed last June and I've never figured out what wpmudev is.

What is wpmudev and how do I use it? As a web developer, using Joomla I have a number of WordPress sites and I am interested in getting better at WordPress, only I have never understood what my wpmudev subscription provides me. I understand Rocket Themes, and I was expecting something along the same structure, only I have received no beginning instructions for wpmudev. I have never figured out how to use wpmudev. I have tried to install it and wpmudev would make my site un-useable, so I've uninstalled it each time. Help me or lose me.