To archive a post, click the ... menu in the upper right-hand corner and click Archive.
Archived posts are recoverable, and can always be found via search. You can see all archived posts by searching is:archived.
To unarchive a post, click the ... menu either in the gray bar at the top of the post, or in the upper right-hand corner of the post, and then click Unarchive.
To delete a post, click the ... menu in the upper right-hand corner and click Delete.
Both archived and unarchived posts can be deleted.
Once a post is deleted, it is not recoverable, so if you prefer to not to delete anything permanently, use archiving instead!