The Plug is a device that converts the A.C. from your 6 volt, 3 watt hub or sidewall dynamo and converts it to D.C. So you can charge most devices that charge via a USB port such as a Garmin GPS or iPhone. Devices it can't charge are those that require 1 amp or more of current, such as the iPad and some phones. For example, the Galaxy S4 apparently ships with a charger that outputs 2 amps. No hub dynamo can produce that much power, so forget about charging the S4 from a bicycle dynamo. This is due to the limited output of bicycle dynamos, not any limitation of The Plug.
The Plug is installed in the steerer tube of a 1 1/8" steel threadless steerer, the most common type used on modern bicycles. It replaces the top cap of the headset.