not too difficult Adele to explain... think there are a lot of techies here.. including me.
the firmware is basically the software/ operating system that "runs" the dvd player.
the dvd player itself is just a piece of electronics.. completely dumb, the firmware is the software on top controlling everything you see, hear.. menus, remote control, how discs are read, you name it.
the vast majority of players still have their firmware fixed and cannot be altered, usually in a chip. a small number of players allow you to upgrade the firmware via a CDR, either sent to you by the dealer or downloaded off the internet. (Toshiba, Philips, Denon.. plus a few others).
so if you encounter a bug with a particular disc then they can fix it just by altering the firmware... you dont have to return your player to have this done since its as simple as loading a CD up.
in theory!