What does "in both houses the number of representatives will be proportional" means?

I'm assuming because this is under the history thread, it's dealing with the U.S. Senate. There are two main political parties in the U.S. which are Democratic and Republican- but there is also an Independent party, just not as big. Representatives are people who represent a certain party in, I believe, the Senate. So when it says "proportional", it's most likely referring to the fact there is an equal amount of representatives who are under the two big parties- the Democratic or Republican.