Routers with a 2.5G Ethernet port are becoming more common, but the port only delivers its full speed under the right conditions. A standard Ethernet port tops out at 1 Gbps, while a 2.5G port supports up to 2.5 Gbps and is usually labeled '2.5G' or '2.5GbE,' sometimes with a different color.
To get that speed, the connected device must also have a 2.5G port, and the cable must be Cat5e or newer. For internet traffic, the ISP plan needs to be at least 2.5 Gbps; otherwise, the plan caps download and upload speeds. The source notes that for most people, multi-gig internet is a nice-to-have, with diminishing returns above 1 Gbps.
Even with a slower internet plan, the port can still help with local network traffic. Tasks like backing up files to a NAS or transferring large files can run at the full 2.5 Gbps if all devices are compatible. The source adds that it will not make streaming or gaming smoother unless the network is extremely busy, since those activities require little bandwidth.