A  radiator air vent (also known as a bleed valve or bleed screw) is a small valve located at the top of a radiator that allows trapped air to be released from a central heating system. 
Purpose and Function
The primary function of a radiator air vent is to maintain the heating system's efficiency by ensuring it is filled with water (or steam in steam systems) rather than air. 
Prevents Cold Spots: Air is significantly less dense than water and naturally rises to the highest points in the system, often accumulating at the top of radiators. This air pocket prevents hot water from circulating effectively, leading to cold patches on the radiator's surface.
Ensures Even Heat Distribution: By releasing the trapped air, the hot water can fill the entire radiator, ensuring uniform heat distribution and making the room consistently warm.
Reduces Noise and Corrosion: Trapped air can cause gurgling or knocking noises in the pipes and radiators. Additionally, the oxygen in the air can contribute to corrosion inside the system's components, which can lead to damage over time.
Maintains System Pressure: A system free of air operates more efficiently and prevents the boiler's pump from overworking due to pressure imbalances, which could cause premature wear and tear or system failure.