The Expire after publish setting on a category controls what happens to a post after it has been published.
Disabled (default)
Posts cycle. After PostRecycle publishes the last post in the queue it starts again from the beginning. This is ideal for evergreen content — tips, quotes, product features — that you are happy to share repeatedly.
Enabled
Posts are used once. After a post is published it is removed from the active queue. Once all posts have been published, that timeslot stops firing until you add new content. This is ideal for announcements, promotions, or any content that should only go out once.
You can change this setting at any time. The change applies to future publishes only — posts already published are not affected.