Fabric is a new platform for building mobile apps
The Fabric platform is made of three modular kits that address some of the most common and pervasive challenges that all app developers face: stability, distribution, revenue and identity.
It seems everyone has to be building a platform these days, but really it sounds like Twitter, having previously shut down developers on their API/platform a while ago, realised that actually letting devs on their platform (now that it includes advertising APIs - thanks to their MoPub acquisiting a while ago) is going to be profitable for them.
Maybe it will be well received, and the way the market changes maybe people will go for it - but I wouldn't be building an app based around a Twitter platform given their track record. The company are a lot older, and maturer now (and they probably released their API too early last time around), but it still seems like quite a risk to have something as central as login/revenue tied to Twitter (I dont like third party logins anyway..).
Anyway, the much more interesting product they released was Digits - which allows sign-in with your mobile number - which as I have ranted before, is an idea that I love, and seems like a really smart move by Twitter. Both in as much that it will move twitter to that space of smartphone is your social-identity/network and also offers a lot more value than just another platform that serves ads and supports login etc.