FTP is the file transfer protocol. That is to say, a means of uploading or downloading files.
FTP is often used because it is fast and easy to use. It also allows you to resume transfers (dependant on the server daemon)
(An ftp server on Unix systems is often called an ftp daemon, or ftpd)
Port 21 is the default port for ftp.
Transfer is either in binary or ascii mode. Some clients can auto-detect on the basis of file extension.

Access may be in the form of a registered user of the system (username and password) or as an anonymous login.
Try logging in via ftp with the username of "anonymous" and password of a valid email address.