Port 23: Telnet
Port 23 is Telnet, an unencrypted remote login used by older network devices.
What runs on port 23
Telnet gives a text command line on a remote device over TCP 23. It predates encryption entirely, so everything, including the password, crosses the network in the clear. SSH replaced it for almost all purposes, but it survives in embedded devices.
On a home network
Some cheap routers, IP cameras, DVRs and ISP-supplied boxes still run a Telnet service, sometimes with a hardcoded password the owner cannot change. Modern computers and phones do not listen on 23.
If port 23 is reachable from the internet
Telnet should never be reachable from the internet. In 2016 the Mirai botnet scanned for Telnet on cameras, DVRs and routers, logged in with a short list of factory default passwords, and built the army that knocked major websites offline in the attack on Dyn DNS that October. Mirai variants still scan for 23 today.
How to check whether it is open on your network
Run a port scan against your router and cameras and look for 23. RECON's Exposure Check tests whether Telnet answers on your public address; if a device has it and you cannot turn it off, keep that device off the internet or replace it.
To find out whether port 23 is open on something in your home, and whether the internet can reach it, the app scans your Wi-Fi from your phone and checks your public address from the outside. It says what it checked and what it could not. Get RECON for iPhone — one-time purchase, no account, nothing leaves the device.