Almost all automotive vehicles use a closed-loop, liquid cooling system. The water pump circulates coolant through the cooling tubes of the radiator, where it cools and travels through the engine passages and hoses. Coolant, or antifreeze, is designed to flow smoothly through all of the cooling passages, with no air blockages. A thermostat opens and closes to regulate the coolant flow for engine warm-up and circulation. Air bubbles in the coolant, at the radiator or expansion reservoir, means air has entered the system at some point. This can lead to overheating.
A radiator cap functions as a pressure seal, and keeps the cooling system pressure raised. It also allows pressure and coolant to vent back to the expansion (overflow) reservoir. A malfunctioning radiator cap seal can allow air to enter the system, often producing bubbles in the expansion reservoir.