The BR100 is a wireless bridge for the Sonos Digital Music System.
The Sonos Bridge (formerly known as the ZoneBridge or ZoneBridge BR100) is a wireless bridge that works with all Sonos audio products, including the Sonos Play:3, Sonos Play:5, Sonos ZonePlayer 90, or Sonos ZonePlayer 120. The $49 unit (formerly $99) doesn't have any audio outputs; instead, it's just designed to pass wireless signals between those aforementioned Sonos systems. In that regard, it can be used to fill in "dead spaces" in a home's Sonos setup. The Sonos Bridge has two Ethernet ports: they can be used to interface with your home network (the Sonos system requires one wired connection), or as a wireless-to-Ethernet bridge for other nonwireless products on your home network (such as an old Xbox 360, TiVo, Slingbox, or Blu-ray player).
To see how the Sonos Bridge can be used to interface with a full Sonos system, read the review of the Sonos Digital Music System BU250.