This is only for the new Android TV devices, like the ADT-1 and the Nexus Player, it is completely useless on other devices
Android TV requires existing apps to be updated and declare themselves compatible before they're installable from the Play store.
While certainly not as streamlined or seamless as the average user would wish for their TV, many apps can still function on some level on these devices if you sideload them, but they don't show up in the Leanback Launcher (the Android TV homescreen).
So for that there is this app, the Sideload Launcher. A simple app that shows you the normal app launcher icons that a phone or tablet would show you, but which are hidden on Android TV, allowing you to launch any sideloaded app!