Ideally, the lower house are representatives elected from the common people, and the upper house are the career politicians that understand how the government works.
In the U.S., the 17th amendment[2] changed that, for better or worse (probably both).
1: https://en.wikipedia.org/wiki/Bicameralism 2: https://en.wikipedia.org/wiki/Seventeenth_Amendment_to_the_U...