Finding Aids are descriptive inventories for archival records and manuscript collections held within an institution. While it is mostly an inventory of a collection at the box or folder level, a finding aid can also give you useful information about the collection, the donor or creator of the collection, and any access restrictions that may have been placed on the collection. A finding aid can guide you to the parts of a collection that will be most useful for your research, and in some cases, such as with Princeton's finding aid, is a way for you to request the materials in advance of your research visit.
Pay attention to publication dates and who published the finding aids and catalogs. You'll also need to be aware of what is not listed, so keep in mind that archival gaps and silences can tell you a lot about the collections you're visiting as well.
To prepare for your archival visits, your best resources are colleagues, peers, and researchers who have most recently visited archives that are of interest to you. You can also look up reviews of the archives and libraries you plan on visiting before your visit.
Archive Reviews on the Dissertation Reviews website is an online resource that contains reports on archive visits written by fellow researchers.
Hazine contains archive reviews for archives in the Middle East, North Africa, Europe, and the Americas and is geared towards those studying Middle Eastern and/or Islamic world history.
Navigating findings aids requires understanding that terminology changes over time. You can refer to Princeton University Library's Statement on Language in Archival Description to learn more.