SFTP is a secure file transfer method, that is supported by FileZilla and many other FTP programs.
When your ISP changes your IP address, and you try to connect the way that used to work, you get this in the FileZilla command section:
Status: Connecting to yourserver.com:18765... Response: fzSftp started Command: keyfile "F:\path\to\keyfile.ppk" Command: open "username@servername.com" 18765 Error: Disconnected: No supported authentication methods available (server sent: publickey)