Search tools like ArchiveGrid and repository websites will often lead you to a guide to the archival collection, known as a finding aid.
Finding aids are the archives’ equivalent of a library book's catalog record or table of contents, and they are an indispensable tool for discovering and evaluating the potential usefulness of archival collections.
Finding aids produced by different archival institutions might vary in functionality or "look and feel" but will include certain common elements or sections.
Here's some information you can often expect a finding aid to contain: