The ATT BGW210 is a gateway, which is a single device that functions as both a modem and a router. It connects to your internet service, converts the signal, and then broadcasts that connection wirelessly (Wi-Fi) or through Ethernet cables to your devices.
Modem function: The "modem" part of the device connects to AT&T's network (like DSL or fiber) and is responsible for bringing the internet signal into your home.
Router function: The "router" part creates your home network, allowing you to connect multiple devices through Wi-Fi or its four gigabit Ethernet ports.