At sea level there is a pressure equivalent to 10 meters of water pressing down on all of us all the time. This is because of the weight of the air above us in the atmosphere. When you travel up a mountain, there is less air above you in the atmosphere. A similar and maybe more obvious situation occurs in water–the deeper you go, the more you feel the pressure of the water above you.
The important effect of this decrease in air pressure is this: in a given volume of air, there are fewer molecules present. This is really just another way of saying that the pressure is lower. (This is called Boyle’s law.) The percentage of those molecules that are oxygen is exactly the same at every altitude: 21%. The problem is that there are fewer molecules of everything present, including oxygen.
At 0ft, the standard barometric pressure is 101 kPa (760 mmHg). This means that there is 100% of the oxygen available at sea level.