What is the 1" Hex shaped thing on the top of my water heater?

It could be a pressure relief valve that is leaking,If so shut off water and open a faucet ,remove and buy a new one of the same rating at a hardware store and replace. There also is a hex headed deal that is about flush with the top of the water heater which is the anode rod which screws down into the heater as bill was stating in his answer before me.