The AVM ISDN-Controller FRITZ!Card USB v21 is a device from AVM, a German company known for networking and telecommunication products. The FRITZ!Card USB is an ISDN (Integrated Services Digital Network) controller that allows users to connect their computer to an ISDN telephone line for high-speed internet access, voice communication, or faxing. The version you're referring to, v21, likely indicates a specific model or software version of the product.

The driver you're asking about is the software that allows your operating system (in this case, Windows) to communicate properly with the FRITZ!Card USB device. It makes the ISDN functionality work, such as internet connectivity, voice calls, and data transmission over an ISDN network.