What it's for
Port 20 is the data channel used by FTP in active mode — the counterpart to port 21's control channel. While port 21 handles commands (login, directory listing, file requests), the actual bytes of a file being uploaded or downloaded flow over a connection using port 20, initiated by the server back to the client.
Protocol basics
Port 20 runs over TCP, since reliable delivery matters for file integrity. In active-mode FTP, the server opens the data connection from port 20 to a port the client specified — a pattern that frequently clashes with client-side firewalls and NAT, since it requires an inbound connection from the server's perspective on the client's end. This is precisely the friction that made passive mode (where the client initiates both connections, using a dynamically negotiated port instead of a fixed port 20) the default in virtually all modern FTP clients.