What does it mean when the air in airplanes are pressurized?

The air in an aircraft needs to be pressurised so that the people within the cabin don't pass-out from oxygen starvation at higher altitudes. The atmosphere can be described as a load of blankets wrapped around the world, the bottom layer has all the weight from above and is squashed, but as you go further up, there is less pressure on the blankets. this is the same with air... at sea level there is plenty of oxygen in the atmosphere to breathe, the higher you go the more sparse the air becomes and less oxygen is available for breathing. Aircraft are usually pressurised to the equivalent of 8,000ft as this is a safe threshold for breathable air.