Port 123: NTP
Port 123 is NTP, which keeps the clocks on computers and devices in sync.
What runs on port 123
Time servers answer on UDP 123 and devices query them to set their clocks, which matters for TLS certificates, logs and logins. Public pools like pool.ntp.org and vendors like Apple, Google and Microsoft run large NTP services. Some devices also run an NTP server so others on the LAN can sync to them.
On a home network
Your phones, computers, TVs and cameras send NTP queries out to the internet, which is normal. Some routers and NAS boxes also run a local NTP server for the LAN.
If port 123 is reachable from the internet
A device answering NTP from the internet is not dangerous in itself if it only answers time requests. Older NTP servers that answered the "monlist" command were abused in large amplification attacks in 2014, turning a small spoofed query into a far larger reply. Unless you deliberately run a public time server, nothing at home should answer on 123 from outside.
How to check whether it is open on your network
Check whether your router or NAS has an NTP server option enabled for the WAN side. An outside scan of UDP 123 against your public address confirms it.
To find out whether port 123 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.