A segmented transfer is transferring a file using several connections at the same time, each connection getting a part of the whole file.
It can make transfers 10 times faster when the speed limit of a single connection is lower than your overall speed. That happens in fact very often.
Add-ons for Firefox like this one:
Work very well in that aspect, and doing the same within an FTP client makes it much more valuable.
Unfortunately, FTP clients that use that technique are rare. The ones that have a good GUI don't, and the ones that do have some other problems so I'd like very much WinSCP to implement this...