​​What is Cabin Pressure and Why do we need to maintain it in Aeroplane ?

Earth's atmospheric pressure varies with altitude. At the Earth's surface(sea level) , It is measured at 14.7 pounds per square inch(P.S.I) - which human beings are accustomed to because it provides the optimum amount of Oxygen.
As Aeroplane ascends to higher altitudes, the atmospheric pressure decreases. There isn't oxygen to breathe and humans cannot survive more than a minute at such altitudes.In order to prevent this from happening , Cabin Pressure is maintained at appropriate levels.