The term rain shadow refers to?

It refers to the blocking of rain on one side of a mountain range. The Sierra Nevada mountains, for example, receive a lot of rain on their western slopes off the Pacific Ocean, but the clouds do not travel across their peaks as well, so little rain falls in the big desert east of this range.

Just a few million years ago, when the range was much lower in elevation, there were big lakes in Nevada which are now arid playa.