What is the main argument in this quote? I have no clue what it even means.
"The senate is truly a chamber of sober second though. To perform that role properly, this chamber should be above the partisan factions of the day. This chamber is supposed to be above the day's fads and fantasies, and it is supposed to be independent."
The main point of this quote is that the Senate should be able to remain above the political excitement of the day. It should be able to sit back, not get swept up in the excitement, and be more rational about things.
What the quote is saying is that the Senate should not really get into taking sides--one party against the other. They are supposed to be the adults--they are supposed to be thoughtful enough to do the right thing, not just whatever the people think they should do.
This was one of the major reasons for having a Senate -- it was supposed to calm the people down and do the right thing.
This quote is a description of the role of the Senate, of course; its purpose is clearly to distinguish the role of the Senate fromthe role of the House. A quick review of Congress may be helpful to fully understand the quote.
Congress is made up of two bodies, the House of Representatives and the Senate. Each state is divided into districts, and Representatives to the House are selected by the voters in these districts in each state. They are elected to two-year terms, and there are various numbers of Representatives per state based upon population. This is supposed to be the legislative body which is closest to the people, as they have to answer directly to their constituents more often than Senators do. (And in today's world where elections start nearly two years in advance, it's almost time to start running for re-election as soon as they're elected.) because of that, though, they are highly motivated by the politics of decision-making. The House can create laws and weigh in on many issues, but it's referred to as the "lower house" because it just doesn't have as much legislative authority or power.
The Senate, on the other hand, is known as the "upper house." Each state only gets two Senators, thus the representation is more equitable. A Senator serves for six years, giving more stability to this legislative body. The Senate alone has the power to determine punishment for Presidential wrongdoings--including removal from office--to ratify treaties, and to approve Presidential appointments. Clearly this is the more powerful of the two houses.
Given that, I think the quote makes perfect sense. The Senate is to be the more reflective and serious body; and, because they serve a longer term, they should not be as concerned with the politics (or partisanship) of their decisions as Representatives are. They are to be the calm deliberators, weighing any bills which they are given from the House as well as the most significant decision-making tasks which I listed above. I'm not sure this is always true in practice, but this quote is true in the ideal.