The Nintendo Account is what the company is using going forward instead of the old Nintendo Network ID (NNID) system. It's what all their new consoles and mobile efforts will connect to.
As a PSA go grab your preferred username before someone else does!
And then go "pre-register" for the new Miitomo app to earn some of the new Nintendo reward currency on your account. The new Nintendo Platinum is taking the place of the old Club Nintendo system.
I'm really excited to have rewards back from Nintendo. They've hinted all the rewards will be digital, but that's fine with me. Discounts and money back for digital purchases would be great.