Well the quickest way to explain would be to send you over to their official NBI website. But for the inherently lazy amongst you (I can't be the only one) here's a quote from their About page.
"Newbie Blogger Initiative is a grassroots community effort to support gamers who want to blog, vlog, stream, and podcast their passion for video games."
In it's most simple form that's really it - a group of mostly like-minded bloggers who want to support and encourage aspiring bloggers from the gaming community.
Starting a new blog is difficult. I started writing this one nearly years ago, basically as a way of keeping friends in touch with what I was doing, somewhere to keep the roleplay stories I'd written for games, and a platform to rant now and again. It didn't matter to me whether anyone else actually read it or not (and to be honest I'm still not that bothered about the "hits" - which is why there's never been any advertising on the blog). I wrote and continue to write it for myself.
Still, I had no idea what I was doing, there wasn't an NBI back then, and the best way I found to get ideas about how and what to write was to read other blogs. Lots of them. Finding the right blogs to follow wasn't always easy. That's where the NBI can help - by giving you a pool of people in the same boat with whom to share advice and ideas.
The other thing the NBI can do is to help you build an audience. I don't believe you should ever really chase an audience, but should write for yourself first and foremost. However it can be dispiriting when starting a new blog if no-one ever shows up to read it or even knows it's there. NBI promotes awareness of new blogs and gives you a ready made audience of friendly people who are already interested in the subject of gaming.
The other reason I support NBI is purely selfish. For a variety of reasons I have far less time to write now than I used to, as anyone who follows the frequency of my posts can attest! Work is busier, I have two young kids, and the little time I do get to play games now doesn't leave much for anything else.
What I do still enjoy is reading gaming blogs though, and the NBI gives me an easy way of finding even more great things to read during my downtime.
So, if you've ever thought about starting a blog there is no better time to do so. The NBI website is full of great advice on a whole host of subjects, much of which is a great read in and of itself.
You can find everything you want to know about the Newbie Blogger Initiative here.