I have a 1.50 K/D, but I like messing around more and more after getting bored with MW2, so it dropped a lot. All I do now is play hardcore with the FAL or snipe on Mercs team deathmatch with an Intervention.
On topic: A hacked lobby is a MW2 lobby hosted by someone with a JTAGed Xbox, which allows the running of unsigned code, much like a mod chip for older systems. A file inside the MW2 .iso, or disk, is modified to give outrageous XP amounts in one kill, and can do all sorts of things, not just modify XP. It can unlock all camos, and complete all challenges for example.
JTAGs are not just used for MW2 lobbies. It can be used for anything, from free Xbox Live Arcade games, to modding other games, like Halo.
I would only recommend joining one if you are truly bored with MW2 and don't care about it too much.