The beginning of a good finding aid will also list the subjects (topics or individuals or locations) that are especially prevalent in the collection.
After all of this, the archivists describes the contents of each box and folder. Finding aids are rendered in various degrees of detail, which can be challenging. If an archivist was given a substantial amount of time to process a collection, they usually include more detail about what's in each individual folder. Usually, though, a finding aid only tells you the label of the folder. It might say something like "Very Important Woman's Correspondence with Labor Activist," or "Debutante Balls, 1918-1919." As a researcher, you don't know if there's one letter or one hundred letters in that folder, or if "Debutante Balls" refers to parties thrown by the Very Important Woman or parties she attended. When you open that folder, you have no idea if you're in for some fun or some frustration.
So, how do you find a finding aid? Sometimes it's as simple as Googling "Very Important Woman." More often I use a database called Worldcat to search through many library catalogs at once. It can be unreliable because each library has to sync their catalogs up with Worldcat; if they do not, you might miss something. To be thorough, I also search the catalogs of libraries and archives that would be a logical place for my historical subject (either an individual or institution) to deposit their records. I look at all the public and university libraries in their city, or I look at special archives dedicated to a defining characteristic of my historical subject (like at an archive that specializes in American Jewish history or one that collects materials from social welfare institutions). It's time consuming to go to each library's website and type your search terms into their catalogs, but sometimes you find complementary collections to the one your were looking for. That's always a nice surprise!
Once I find my finding aids, I then decide if a collection seems valuable enough to justify a visit to that archive. Sometimes it's an easy decision, like when a library has the complete papers of the Very Important Woman who is the subject of your dissertation. Other times, you know you want to write a chapter about the Very Important Woman's favorite charity, and you look at the finding aid for their records--which with your luck are at an archive on the other side of the country--and you realize that the charity only donated one box of documents and all of the folders are labeled "Tax Forms" and dated from the 1970s, well after the Very Important Woman died. It's not worth buying a plane ticket to look at that! That's when you decide to cut the chapter, or to base it on research that has already been done (secondary sources).
Finding Aids are an invaluable tool to a researcher, but like all tools they vary wildly in quality and helpfulness.* Sometimes you end up with a top-of-the-line power circular saw with laser precision guide and built-in level, and sometimes you end up with this. The more you use finding aids, the more adept you become at working with what you're given!
If you're interested in looking at a few examples, here are a few finding aids from my own research that represent the spectrum of detailedness:
Solender Family Papers (American Jewish Historical Society/Center for Jewish History)
Fort Washington Branch Records (New York Public Library Archives)
Henrietta Scherer Papers (Tamiment Library, New York University)
*Let me make it clear that this is not always the archivists' fault. They are often underfunded and overworked. Most archives have more collections than their existing staff is able to process, but grants to hire more helping hands are really competitive and hard to come by. I love archivists and librarians, and could not do my job without them.