The House of Representatives has legislators who are elected on the basis of a state's population, while the Senate has an equal number of legislators from each state. The reason is that the framers of the US Constitution argued over which was more fair, an equal number of legislators per state (which seems unfair for states with larger populations) or legislators apportioned by population (which seems unfair to states with less density of people). The compromise was to do both.
Since the British Parliament was a bicameral legislature, ie one with two houses of lawmakers, the concept of two federal legislatures was not so alien. But where the British had a House of Lords (with hereditary membership) and a House of Commons with legislators apportioned by population density, the new American nation had no aristocracy. Therefore the lower house (Congressional Representatives) was apportioned as in the House of Commons, while the upper house (the Senate) consists of an equal number from each state. The population-apportioned House holds the financial reins, as in the House of Commons. This is covered in Article One, sections one, two and three of the Constitution.