A valve stem is a crucial component of a tire that allows for inflation and deflation. It is a small tube that extends from the tire and connects to the valve, which controls the air pressure inside. When you use a pump or an air compressor, the valve stem opens to let air in or out, ensuring the tire maintains the correct pressure for safe driving. Valve stems are typically made of rubber or metal and are designed to withstand high pressure and temperature changes. They can wear out over time, leading to air leaks, which is why regular tire maintenance is important for vehicle safety and performance.