The data channel of classic FTP. Port 21 carries the commands and 20 carries the file contents themselves, which is why FTP needs two connections and why it behaves so badly through firewalls and NAT.
FTP sends everything, credentials included, in plain text. Anyone positioned between client and server can read the lot. Use SFTP (over port 22) or FTPS instead.
Worth knowing. Passive mode uses a high random port for data rather than 20, which is the source of most "FTP works from the office but not from home" problems.
Seeing 20 open usually means a legacy FTP server is running. If it is there for one deployment script, replace that script with SFTP and close both 20 and 21.
The port scanner will tell you whether 20 is reachable on a host you control. Checking from outside your own network matters: a port can be open on the machine and still be blocked at the firewall, and it is the view from the internet that decides whether anyone else can reach it.
Only scan hosts you own or have permission to test.