Answer:
As context, there are two sides of a mountain range:
A) The windward side
B) The leeward side
As mountains are tall, they block clouds which are in the troposphere. All the moisture thus falls onto the windward side, while almost none of the moisture from the rain gets to the leeward side.
In a nutshell, mountain ranges create barriers for precipitation which blocks off moisture to the leeward side, creating deserts.
This is why you see mountain ranges near deserts.
