Periscope is an app which allows you to live stream a video feed from your phone camera to the world.
It's a pretty interesting idea, especially if you feel like watching people typing at work, sitting around at home or sometimes doing something interesting.
The service was snapped up by Twitter before it ever went live in March 2015 so it's not surprising that up to now user have been required to sign up with their Twitter handle. But that's changing a little now as you can also sign up with your phone number.
Much of the social aspect of Periscope still works using Twitter IDs so you'd still be better off having an account before you use it.
Periscope is currently available on iOS and there's an Android beta in the wild at the moment, so a full product should be available quite soon.