Similar to a router, a modem helps you get connected to the internet. The modem allows you to connect to your ISP (Internet Service Provider). Even though most home networks have one device that functions as BOTH a router and a modem, it’s common in advanced setups to have a separate router. In these instances, a modem will commonly be run in Bridge mode so that internet traffic is passed to the separate router.
Think of it in this way:
Outside world <—> Your Modem <—> Your Router <—> Your Devices
