It's the same technology commonly found in a variety of electronic devices such as television remotes. The infrared sources located in the sensor bar emit lights that can't be seen by the naked eye, but are nevertheless plainly visible by the Wii remote, whose tip features a very basic camera that can read IR data.
Where should the Wii sensor bar be?
The Sensor Bar can be placed either above or below the TV screen. Decide which option best fits your television set up. Above the TV Screen: Insert the Sensor Bar plug into the Sensor Bar connector on the back of the Wii console.
Where can I get a Wii sensor bar?
The Sensor Bar plugs into the Sensor Bar connector on the back of the console and is then mounted on or below your TV. It works in conjunction with the Wii Remote for motion detection.