The idea is to create a browser mediated login experience that gives the identity provider and web app what they need without being able to correlate requests across the Internet.
I am working on an article on this topic. If you are interested in learning more, here's a video from a recent auth focused conference (full disclosure: my company put it on and I emceed): https://m.youtube.com/watch?v=FBAD4x7MWdI
They are actively working on the standard and Firefox has committed to it. Edge already supports it. They are looking for identity provider feedback.
More here: https://github.com/w3c-fedid/FedCM (we meet weekly on Tuesdays).