Your best friend for file transfer.Fetch
Fetch displays the warning, "If you do not disconnect, your password will be sent over the network in the clear to a server that supports security," whenever you try to send your password unencrypted and insecurely to a server that says it supports a more secure way to send passwords; in other words, you are not taking advantage of the server's security support.
There are two reasons you could be receiving this warning.
Usually you will receive this warning when connecting to a server that supports Kerberos, and you didn't specify FTP with KClient or FTP with GSSAPI as the connection type in the New Connection dialog when connecting to the server. You could also receive it even when you did select one of those options because the server is not working properly; in that case, you should contact your network administrator. You should never see this warning when connecting to a server via SFTP.
However, you may also see this warning when connecting to a server that supports TLS/SSL, or a security system that is not recognized by Fetch. To avoid getting a warning in this case, either connect to the server using SFTP (if available), or uncheck the Warn before sending password insecurely to Kerberos servers preference in the Security Preferences pane.
Click the Ignore button to continue connecting, or Disconnect to not make the connection.
Consult your network administrator or service provider if you have any question about whether the warning is significant.
You can turn off this warning by unchecking the Warn before sending password insecurely to Kerberos servers checkbox in the Security Preferences pane.
There is also a more general warning that applies whenever you try to send your password in the clear, regardless of whether a server supports security or not.