A frangible bulb sprinkler has a liquid-filled glass bulb that holds the cap on the sprinkler orifice, preventing water flow. In case of a fire, as the temperature begins to rise, the liquid in the bulb expands. When the air temperature gets hot enough, the liquid expands to the point of breaking the bulb.
That lets the water pressure push off the cap. Water flows out of the orifice and hits the deflector. The deflector creates a spray that covers the specified area.