The Post Options are available for all posts, but only your Unpublished posts will have these: Post Now, Reschedule and Make Draft.
Post Now will publish the Queued post in question immediately, if you are not on Cooldown. If you are on Cooldown, it will be set to Publish as soon as it ends.
Reschedule will allow you to change the date and time a Queued or Scheduled post is set to Publish.
Make Draft will let you cancel the posting time and will move the post in the Drafts tab.
On your already Published posts, you will also find one more option, Unpublish. This will make the post unavailable to others and will move it into your Drafts tab.