'Shot' valve?

I was tinkering with my car today, and noticed something i’ve never seen before. There was a tube leading form the firewall to the engine, with a valve on it to open or close the tube. The valve was marked ‘shot’ and had an arrow on it. What exactly is this, and what does it do??
Thanks :slight_smile:

Pretty sure you’re looking at the hose that runs coolant into you heater core…the valve is there to stop the flow of coolant to make it either hot or cold in the car