Boyle’s Law is a law for an ideal gas. An ideal gas is a perfect gas that follows all the gas laws. Ideal gases are hypothetical.

Boyle’s Law states that for a fixed amount of an ideal gas at a fixed temperature, P (pressure) and V (volume) are inversely proportional, which means at a fixed temperature, when pressure increases, the volume will decrease. If the volume increases, pressure decreases.

Example: A jar filled with an amount of gas.

You have a large jar (volume UP) with an amount of gas. The jar is large, so it gives the molecules a lot of room to bounce around (pressure DOWN) inside the jar.

Now you take that same amount of gas and move it to a small jar (volume DOWN). There’s a lot less room for those molecules to move around now, so they’re bouncing all over (pressure UP) the walls of the jar like crazy.

You can see how volume and pressure are inversely proportional. When one increases, the other decreases. 

It’s important to remember Boyle’s Law is based on measurements at the same temperature.