A law that states that under "normal" conditions, the Pressure of a gas is inversely proportional to the Volume while both are directly proportional to the amount of gas (n) and Temperature (which, again, are inversely proportional to one another.)
This roughly means that if Pressure increases Volume decreases, Pressure and Volume tend to increase with Temperature, etc.

The ideal gas law is commonly expressed as PV = nRT where R equals 0.0821 when Pressure is measured in atmospheres, Volume in litres, Temperature in kelvins and amount of gas in moles. If we measure pressure in pascals and Volume in metres cubed, R is set to 8.31.