At sea level there is a pressure equivalent to 10 metres 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,
The important effect of this decrease in 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: 21%. The problem is that there are fewer molecules of everything present, including oxygen.