Quote Originally Posted by JDMJAYDC2
looks like a water jacket yeah i broke one of those too on my old ls trying to bleed the system its crazy how easy they snap off
Your water jacket is what the volume of water/coolant inside the head and block are commonly referred to.

Quote Originally Posted by 1kreationz40
that looks like a thermostat, but im thinking thats your heater assembly, controls hot and cool air.....
Only half right. Evasive was right in his intial post. That is the heater control valve. It controls the flow of coolant into the heater core and allows you to have heat inside the cabin. When the valve is closed, no coolant flow to the heater core and thus no heat transfer.