Open Threads are informal discussion areas, where users are welcome to post comments that didn't quite feel big enough to warrant a top-level post, nor fit in other posts.
Sometimes an Open Thread focuses on a specific topic. The most common Open Threads are the monthly Open and Welcome Threads, which serve as a general focal point of discussion, as well as a place for new users to introduce themselves.
Note: if a post is in the AI Questions Open Thread series, it should get that tag instead of this one.