A modem connects your home to your internet service provider (ISP) and translates the provider’s signal into a form your equipment can use. A router takes that connection and creates a local network so phones, computers, and other devices can share it—usually over Wi‑Fi or Ethernet.
You need both roles for typical home Wi‑Fi. They can be two boxes or one combined “gateway.” A modem alone usually supports only a single wired device and does not create a home wireless network; the router handles multi-device sharing, local addressing, and common security features such as a basic firewall.
Source: Modem vs router: What is the difference, and do you need both? — Verizon