What is the slot on side of iphone 4s for has a key for it?

Has this luttle metal thing that pulls the slot out what's it for

Its to put the SIM (atleast it is called lite that in my country)( i'm not english)

The slot on the side is for your SIM card, the little metal thing is to open it up and take the card out which I recommend you DO NOT do or your phone won't work.

That's the SIM slot where you put the SIM card in. The "key" is actually called a SIM ejector tool.