I think it might be like a region or version id, because if you were to look up say black ops II on xbox unity, there's only one listing for the game. however, there are multiple media id's. I'm assuming the title id is the same for all regions, just the media id is different. I'm not entirely sure though, but I'd say that's a good guess as to what it's for. if that's the case, then it's important or the title update will not work.
I read that you can find media id in information in FSD and Aurora. if not, you might be able to find it with the http connection to your pc. there's a user name and password that you are tied to it (think the default for both is xbox, but I can't remember). you can change them (both will be visible when you go to change them, so you'll see what the default ones are). enable the connection btw. then, type in the ip address of the 360 in the address bar of your pc's web browser. you can do several things in here, such as change artwork, remove things from the hidden folder (i.e. other discs you may have hidden from multi-disc games), etc. I know it has title id at least, so I assume it has media id as well.
