what does "steam key" mean?

[Upatdawn]

seeing PC games for sale, i suume with no seriel key?

[IanH755]

A "steam key" is a multi-digit code you type into Steam which shows you've bought the game (from somewhere other than Steam) and, once Steam authenticates that its valid (usually within seconds) you can install the game.

More info - https://help.steampowered.com/en/faqs/view/2A12-9D...

[Bluemondy]

A serial key you out into steam and you can download your new purchase