That's one of the heater core hoses. You should have a loose hose w/ a heat control valve in the hose somewhere close by.
Also you should be losing coolant from that nipple unless someone has also blocked the return hose somewhere. It is possible someone routed the removed hose to the return side on the engine and both hoses are bypassed.
This did not happen accidentally. Someone must have removed for some reason. Did you recently have work done or purchase the truck?
Ummmm, the hose that's directly under that open port is the one that goes on it. It's a vacuum line, and it reduces down to the metal line that heads into the intake tube between the airbox and throttle body.