A USB Dongle (or just Dongle) is a small plastic piece that plugs into a USB slot on your computer. The Dongle contains registration information for your Bytescribe software.
As long as the Dongle stays plugged into your computer, the software is registered. If the Dongle is unplugged from a computer, the software becomes unregistered and cannot be used on that computer until the Dongle is re-inserted.
If you need to move the software to a new computer or re-register after reinstalling Windows, just make sure that the Dongle is inserted and that the drivers are loaded, and the software will be automatically registered.