The Nokia USB Modem 21M-02 Wireless AT Device is a mobile broadband USB modem that allows you to connect to the internet over a cellular network, typically 3G or 4G LTE, depending on the device's capabilities. It works by connecting to a USB port on your computer and using the mobile network to provide internet connectivity. The Windows driver is required to enable your computer to communicate with the modem and to use it as a network device.

The AT Device part of the name likely refers to the AT command set, a standard used for communication with modems. The modem can be controlled through AT commands for dialing, connecting, disconnecting, and checking the connection status, among other things.