Port 20: FTP data
Port 20 is the data channel for FTP in active mode.
What runs on port 20
Classic FTP uses two connections: commands on 21 and file data on a separate one. In active mode the server opens the data connection from its port 20 back to the client. Passive mode, which most clients now use, avoids port 20 and uses a random high port instead.
On a home network
You rarely see 20 listening on anything. It appears alongside FTP servers on NAS boxes, routers with USB storage and some scanners.
If port 20 is reachable from the internet
Port 20 matters only as part of an FTP server, which sends files and passwords unencrypted. If you see FTP on your public address, the real problem is port 21 and the server behind it.
How to check whether it is open on your network
Look for an FTP server option in your NAS and router settings and for forwarding rules covering 20 and 21.
To find out whether port 20 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.