"'Yo' is a single-tap zero character communication tool. Yo is everything and anything, it all depends on you, the recipient and the time of the Yo," according to the app store description.
Developed by San Francisco-based Life Before Us, LLC, the app's creators describe it as "the simplest & most efficient communication tool in the world."
And it is really as simple as it sounds.
Download it, choose the friends you want to send your "Yo" greeting to, and they get a push notification on their phone and that's it. Seriously.