Explanation:

Have a friend put their hands on your shoulder and push down. Now try to jump upwards. It's difficult to do so right? You would need more energy to jump higher correct? The same applies to molecules of water. The air pressure exerts a force on them which they must overcome in order to boil. The greater the air pressure the greater the temperature at which the water will boil. Hence, the lower the air pressure say up on a mountain, the lower the boiling point.