A tenth prestige lobby is where a hacker/haxor creates a private match where a mod menu is displayed, commonly containing a feature where you can convert from whatever level you are, to 10th or 11th Prestige Level 70. It also can give you an aimbot, wallhacks, all challenges, all titles and emblems and a modded name/clan tag. You need ISO mods or a Jtag to do the lobby. A Jtag is a cable used to use modding software on your Xbox 360/Playstation 3. Also what every Modern Warfare 2 has regretted getting into.
John: Dude i got 1600 microsoft points!
Jim: I know someone who will do a 10th lobby for you!