There are two ethernet ports at the back of your eero.

Your gateway eero will use one of the ports to connect to your ONT fibre modem, leaving you with one more.

You can either use it to provide a wired connection to another eero elsewhere in the property or to provide a wired connection to another device, such as a PC or Smart TV; it's totally up to you.

If you need to connect multiple devices, then maybe you'll use it to connect an Ethernet switch.