A two house legislative system was created to make sure that states were equally represented. For example, in the senate there's two people per state, which means that a state with 1000 people has the same amount of people in the congress as the state with 10000000 people, meaning that the 1000 could decide for the 10000000. To avoid this, there's two houses, one like the senate where there's equal representation. And the house of representatives which is based around the amount of people per state, decided every decade using a census. The number of representatives thus changes every 10 years.
