Basically, it says 'don't get hung up on the rules, just have fun.' The Introduction is, for the most part, what you'd expect if you've played D&D for a while a brief explanation of what a Dungeon Master is, how it involves varying levels of being an inventor, writer, storyteller, actor, improvisor and referee, and that the rules are meant to help the game, rather than put everything into tiny inflexible boxes.
Frist, though, we get the Introduction section. 320 pages including the index, so a fair bit to work with. Getting ideas is why this book was pushed at me, after all. Some bits might be short, and others will probably involve long, rambling tangents as they give me ideas. Take a good long look at this book, and write up my thoughts as we go along. As far as I can tell, the Dungeon Master's Guide doesn't usually get looked at too much - the only one I could find was the 1st edition one - so I figured I might as well combine the two. So, I have been given a reason to read this in-depth, rather than simply use it to look things up as needed, and I've also recently been quietly reading the other Let's Reads going on/that have happened.