Pardon if this sounds dumb but what is TF2? When I see that I think Team Fortress 2 but that's free to play on Steam so I'm thinking it is probably some other game I'm not familiar with but possibly missing out on perhaps. :)
Yes, it means Team Fortress 2. A TF2 key is common currency among Steam traders; they don't mean "a TF2 Steam key"; instead it's an in-game item. I have absolutely zero clue what they are meant for in game; you probably open a crate with one or something and get some item, but lots of people opt to sell them instead. The price fluctuates constantly between $1.5-$2, and the profits go directly to your Steam wallet.
edit: Oh and another name, the official one, for TF2 key is Mann Co. key.
A Mann Co. key sounds so dirty and funny in japanese :P