FTP

FTP (File Transfer Protocol) is a protocol that is typically hosted on port 21.

It facilitates file transfer between client(s) and server or directories of web server.

FTP enables authentication with username and password, so it may be vulnerable to brute-force attacks.

Login with FTP

After you have discovered that the host has a ftp port, you may try to connect there with ftp:
ftp target-ip

It will ask username and password, and if successful you may enter the OS and operate with its shell.