about:mod queue
The moderation queue (usually referred to as the "modqueue") is a system used to filter out images that do not meet Danbooru's quality standards.
All posts by non-Contributors start out as pending and go through the modqueue. Approvers look through the queue and approve posts that meet Danbooru's standards. Post that aren't approved within three days are deleted.
Contributor-level users have their posts automatically approved, bypassing the modqueue. You may be granted this privilege by consistently uploading high-quality posts while maintaining a low deletion ratio. The general criteria for Contributor level is having 500-750 uploads and having a recent deletion ratio of around 5% or lower, though users with very few deleted posts might be promoted at lower thresholds.
If a post is deleted and you think it deserves a second chance, you can appeal it. See howto:appeal for details. You can also ask for feedback in the Upload Feedback Thread. On the other hand, if you think a post is poor quality and shouldn't have been approved, you may flag it to return it to the modqueue. See howto:flag for details.
You can view the posts currently in the modqueue by searching status:modqueue. Approvers may use status:unmoderated outside of the modqueue interface to see the pending posts they haven't made a decision on.