The 11 General Orders of a Sentry
1. To take charge of this post and all government property in view. 2. To walk my post in a military manner, keeping always on the alert, and observing everything that takes place within sight or hearing. 3. To report all violations of orders I am instructed to enforce. 4. To repeat all calls from posts more distant from the guard house than my own. 5. To quit my post only when properly relieved. 6. To receive, obey and pass on to the sentry who relieves me, all orders from the Commanding Officer, Command Duty Officer, Officer of the Deck, and Officers and Petty Officers of the Watch only. 7. To talk to no one except in the line of duty. 8. To give the alarm in case of fire or disorder. 9. To call the Corporal of the Guard in any case not covered by instructions. 10. To salute all officers and all colors and standards not cased. 11. To be especially watchful at night, and during the time for challenging, to challenge all persons on or near my post, and to allow no one to pass without proper authority.
What it actually means
These are the rules for standing a watch — the sailor and Marine version of "here is your job while you are the one guarding this." Recruits memorize all eleven word-for-word in boot camp, and drill instructors will test you on any of them at random, by number. The wording above is the Navy version; the Marine Corps version differs mainly in the sixth order (it names the "Officer of the Day" and "Non-Commissioned Officers of the guard"). General Order 5 — "to quit my post only when properly relieved" — is the one that becomes a life-long reflex.
Where it came from
The eleven general orders are a long-standing tradition of the sea services and have been standardized in Navy and Marine Corps regulations for over a century. The Coast Guard uses them as well. The Army and Air Force distilled the same idea down to three.
U.S. Navy / U.S. Marine Corps standing orders for sentries — reference