The formula P1V1 = P2V2 represents Boyle's law. It states that the pressure and volume of a gas are inversely proportional to each other, at a constant temperature. This means that as the pressure on a gas increases, its volume decreases, and vice versa, as long as the temperature remains constant. Conversely, if the pressure decreases, the volume increases.
For example, imagine you have a balloon that is filled with a fixed amount of gas. If you were to squeeze the balloon, you are increasing the pressure on the gas inside, and the volume of the balloon would decrease in response. Conversely, if you were to release the pressure by letting go of the balloon, the volume of the balloon would increase as the pressure on the gas inside decreases.
Therefore, if you know the initial pressure and volume of a gas, and the pressure is changed while keeping the temperature constant, you can use Boyle's law to calculate the new volume of the gas.