It is a question that gets asked a fair bit so thought I would share more about it.
As explained above by Defure, it is when a period of time has gone by without a thread being posted on. Generally around several months to even years without new replies, and then someone posts on the thread which ends up bringing it back to the most recent threads on the thread list page. An example of this would be a thread that last had a reply from March 2019, and then someone posts on that thread today in February 2023.
It is not against the rules to necropost on these Forums and it is fine to bring back old topics. As long as it continues the discussion or adds something new to it, it is fine if you encounter an old thread and want to discuss on it. It can trigger new discussion from other members as well.
The cases where it isn't allowed is when it is not related to the topic, or just posting to state anything along the lines of "this thread is dead" or "wow this thread is old". It becomes disruptive this way and doesn't actually help to bring back an older topic for discussion.
