Auto Ship is a feature that will automatically send your book to print in 7 days once it has 60 pages in it. Your book will only head to Auto Ship if you have opted into this feature in your Series subscription and you have an available credit to go towards the Auto Ship for your next book.
To turn Auto Ship on, go to your Series subscription and click into the Settings' Wheel in the upper right-hand corner of the page. Scroll to the bottom of the page and click "Auto Ship". Click ON and "Save Changes". This prompt will look like this:
You now have Auto Ship turned on for your subscription.