When you go up to a high altitude the air is thinner so there is less oxygen. Just walking around can make you out of breath like you just ran a mile. After a while some people will get altitude sickness which makes you nauseous and tired, gives you a headache etc. If you go up too quickly you can also get something called HAPE (high altitude pulmonary edema) which is when your lungs fill up with fluid and you essentially drown. This is what happens to some high altitude mountain climbers.

You don't get high though, not even a little buzz. If you smoke weed at high altitudes, however, you do seem to get a lot higher. Not sure why.