Network settings for Proxies

What does "Use passive FTP mode (PASV)" do if enabled, in the advanced Network settings, under Proxies? This is enabled by default so I'm guessing it's for system services, but there's no info about it when I clicked for more info from the panel.

Thx

Update: Instead of getting the ol' LMGT4U, I searched google and actually found info that this mode is sometimes not recommended as it may not be secure. Is this correct and applicable to macs. I use cyberduck for ftp. Thoughts on both this Passive mode setting and if there's a better ftp app, that's cheap or free. I used to use Fetch, any difference there?

Thx again!