A modem is a device that sends information between the outside world or Wide Area Network (WAN), and your home.
A modem turns your inbound connection (coax cable, telephone line, fiber optic line, or other) into an Ethernet connection, which then allows a Wi-Fi router to connect to the Internet.
Modems are usually provided by your Internet Service Provider (ISP). Your router or primary Google Wifi point should be connected to your modem with an Ethernet cable.
Modem and router combination devices (Gateways)
Many ISPs provide a device that includes both modem and router functionality. Connecting your Nest Wifi router or primary Google Wifi point to these types of combination gateway devices is okay. But for optimal setup, you may need to adjust settings on the combination device to ensure your Nest Wifi router or Google Wifi point is your network's primary router. Learn more about Double NAT.
It is recommended to turn off the built-in Wi-Fi in your combined modem and router device. If turning off the Wi-Fi functionality isn’t possible, change the original Wi-Fi network name to something else. Afterwards, set your Google Wifi or Nest Wifi network name to what you used previously with your old modem and router device. By reusing the network name, you won’t have to reconnect devices that were previously on the network.
Don't have a modem?
Some apartment buildings and college dorms don't require a modem for Internet access. You can plug a Google Nest Wifi router or Google Nest Wifi point directly into your wall's Ethernet jack.