I’ve seen it been called Google Fit as well.
It’s simply an app that tracks how much you move. Walk etc.
It’s a very clean looking app that seems to be more geared towards to know just how much you’ve walked etc during a day than to be full sports tracker (Like Endomondo etc). Haven’t tried out Google Fit yet, but I’ve installed it on my phones and will test it out over the next week or so.