The less air pushing down on the water effectively allows the water to boil easier. Think of the water molecules as a bunch of marbles bouncing around in a box with a lid. The less air pressure means the lid is easier to open from the force exerted by the bouncing marbles, allowing those particles to escape and turn into gas.
So in short, if you are in the mountains, then it'll take less time to cook food because water comes to a boil faster.
