Including tablets, smart-phones, TVs, and Apple TV devices. Essentially the app will enable streaming via a via 5Ghz network or wired Ethernet. Which means you'll need a pretty modern home-network to make the most out of it. The Steam Link App will feature support for the Steam Controller and is expected to launch the week of May 21.
With Valve noting that Android access will initially be offered in a Beta state. A second Steam Video app will be made available at a later date that will provide access across multiple devices to steam's growing library of movies and shows. Which, in case you forgot, is still a thing.