The House of Representatives is more representative of the needs and wishes of all the people they represent. In the Senate, there are two senators for each state no matter how many people live there. Smaller states get much more power because they have as many senators as very crowded states.
For example, Los Angeles county (just Los Angeles!) has over 9 million people. That is more people than Montana, Wyoming, North Dakota, South Dakota, Maine, Vermont, New Hampshire COMBINED. But in the Senate, California still only gets 2 senators. All of those other states with tiny populations get 18 senators total, so they can outvote California.
On the other hand, the House represents districts/cities/towns. More people = more districts = more representatives This way, states with more people have more of a say in politics.
A large state like California has 53 representatives. A small state like Wyoming only only has one representative. This system allows big states to have more power.