People asking for keys...

CS:GO or TF2 keys (in-game items) that are sold on the steam market and cost about 2 dollars each, that's pretty much all I know about it as I'm not really into those games, but that's pretty much all you need to know about it actually.

CSGO/TF2 usually. Keys used to open chest drops in those games.

You can buy these keys on the market too (e.g. Chroma 2 Case Key).

In-game keys from either TF2 or CS:GO

Dota 2 has keys too.

Since you question has already been answered, Welcome to the site :)

If it is any consolation, you are not the first to be confused by it as a starting trader, nor will be the last. It really doesn't help that a large portion of the trading is for Steam game keys (as in game activation codes) now, so it can totally mess up any description.