Two questions are immediately worth addressing regarding the device: what is it, and how does it compare to Google's Chromecast?
In short, the Streaming Stick (which was announced in March) is a small dongle that plugs into an HDMI port in the back of a TV to provide the user with a host of "channels" streamed from the internet. Netflix, Now TV and BBC iPlayer are some examples of the channels available.
The basic premise is that serving such internet-based content to a living room TV provides a better viewing experience to users, than having to watch content on a laptop. Roku provides a remote control with the Streaming Stick, to make the experience as similar as possible to normal couch-based viewing.