Gay-Lussac's Law states that the pressure of a gas is directly proportional to its temperature when the volume is held constant. This means that if you heat a gas, its pressure will increase, and if you cool it down, the pressure will decrease. This relationship is important in understanding how gases behave under different temperature conditions. For example, if you have a sealed container of air and you heat it, the air molecules move faster and collide with the walls of the container more forcefully, increasing the pressure. This principle is crucial in various applications, such as in automobile engines and pressure cookers.