Safety valve
What is safety valve in coffee?
A safety valve is a small pressure-release valve on the side of a moka pot base. If the pressure inside builds up too high, for example because the coffee is tamped or ground too fine, it opens and lets steam out. Always fill the water to just below it, never above, and never block it.
A valve that hisses during brewing is a sign to grind slightly coarser, fill the basket level rather than packed, and check the water is below the valve. See the moka pot guide.