The correct expression of Boyle's law is "P1V1 = P2V2". This law states that the product of the pressure and volume of a gas at a constant temperature is always constant. In other words, if you change the pressure of a gas, its volume will change in an opposite direction so that the product of the pressure and volume remains constant.
For example, if you increase the pressure of a gas, its volume will decrease, and vice versa. This relationship between pressure and volume is useful in many real-world applications, such as in the design of gas storage tanks and gas transport pipelines.