The short, sweet, and simple definition for FTP is File Transfer Protocol, which allows you to have a simple way of transferring files between computers (or in this case, an XBox to your PC and back). With this being said you should get yourself a few essential materials for this.
A Crossover Cable (cheap and can be found at most electronics stores or made if you have the gear)
An FTP Program (i use Flash FXP
but whatever flavor you like works)
A modded XBox (mine is softmodded with the tutorial that i published
If you have XBMC installed as your Dashboard, you can find out your XBox IP address in the settings section (once you have connected your XBox to a PC via Ethernet with a Crossover cable). After you set it to be online (not hard trust me.. if you soft modded your XBox you can do this part yourself). Then simply open your FTP program, then in the section that allows you to set a connection, simply enter the IP address from the XBox (write it down and stick it on a post it on your computer), then enter in 'xbox' for username and password (that is the default UN and Pass for the Xbox). then its a simple matter of transferring the files from the directory on your computer they are saved on to the are on your PC to your XBox they go to usually on the E Drive (under the Dashboard folder for softmods). Then you can load the files in the corresponding subfolders (usually under Plugins, there are ones like Video, Audio, Programs, Emulators etc). You can usually find some of these folders in forums and with simple Google Searches.