Its a coolant quick disconnect. They are known to break off in the manifold because they get brittle over the years. If you get lucky and it comes out in one piece go to lowes or home depot and get a brass barbed fitting to replace it. It will last alot longer than that quick disconnect fitting. And you'll need some heater hose also. I forget the thread size but I believe its 1/2 or 3/4.
Here is what the part you need looks like if you want to go stock again