Port 21: FTP
Port 21 is FTP, an old file transfer protocol that sends passwords in plain text.
What runs on port 21
FTP servers accept commands and logins on TCP 21, and move file data over a separate connection (port 20 in active mode, or a random high port in passive mode). It dates from the early 1970s and has no encryption unless the server supports FTPS. SFTP, which runs over SSH on 22, is a different and safer protocol.
On a home network
NAS boxes, routers with a USB drive attached, some printers and scanners (for scan-to-FTP) and old IP cameras can run an FTP server. It is often off by default.
If port 21 is reachable from the internet
Exposing FTP to the internet means usernames, passwords and files travel unencrypted, and FTP servers are brute forced constantly. Servers that allow anonymous login have leaked large amounts of personal data over the years. If you need remote file access, use SFTP, a VPN, or your NAS vendor's encrypted remote access.
How to check whether it is open on your network
Look in your NAS and router settings for an FTP server option, and check port forwarding for 21. RECON's Port Scan will show which devices on your network answer on 21.
To find out whether port 21 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.