Basically, it concerns the ID and status of each: send-campaigns, bounce, and feedback loop processing. You can remove related PIDs if they are hung to make sure they can go again and reset the status when they were stuck.
Beyond what it says directly at the emergency actions page, it is covered here to some degree:
https://kb.mailwizz.com/articles/seems-that-bounces-arent-processed-what-can-i-do/