A WiFi modem is a piece of equipment that can potentially allow you to use Google WiFi in your home or office. It is a WiFi device that communicates wirelessly with the Google WiFi equipment installed on lamp posts and serves a function similar to a DSL or cable modem. Like a DSL or cable modem, it requires power and has an Ethernet connection to which you can attach a PC, laptop or personal home router. In short, a WiFi modem allows a user to convert the Google WiFi wireless signal into a wired signal.
What makes a WiFi modem distinct from a conventional WiFi client is that it has transmit power capable of reaching the WiFi equipment installed on lamp posts, and software features that are optimized for use with an outdoor network.