A finding aid for a collection of manuscripts or archives is the best tool available for finding out what's in that collection. It is found online and has a lot more detailed information than can fit in a regular library catalog record.
It gives you a brief overview of the collection, where it came from, who created it, and then a list of what is in it, usually at the folder level.
Finding aids can help you plan your research - for example, if the collection you are interested in has 100 boxes, you might use the finding aid to narrow down exactly which boxes are of the most interest to you.