It's a freewheel. Park Tool sells the splined tool that goes inside there. They're around $6 where I live.
Unless you wish to dismantle the freewheel, do not get a pin spanner to unscrew the top race where the two little divots are. I just happen to like to disassemble freewheels and have done it many, many times. I wait to get the freewheel off the hub first. Although I have disassembled, de-greased, put new ball bearings, top & bottom, while the freewheel was still attatched.
It is not for the impatient. It helps to have small fingers and a steady hand.