Organize Series (apparently rebranded as PublishPress Series) is a plugin for WordPress that allows users to organize related posts together so readers can easily discover the related posts.
For example, I recently did a series of posts on a line of DC Comics greeting cards from the 1970s. Since there were so many such greeting cards, I divided these into five separate posts.
Someone discovering one of the posts via Google, etc., may not easily find the other four posts. With the Series plugin, I can make that more visible to visitors like this:
PublishPress recently purchased the plugin and has a series of “Pro” enhancements that adds shortcodes, the ability to add a post to more than one series, etc.