It was a reference to the secret signal orchestrated by Revere during his historic ride from Boston to Concord on the verge of American Revolutionary War. The signal was meant to alert patriots about the route the British troops chose to advance to Concord.

https://qanon.pub/data/media/b0bea38c7dc62095cd6fa6f8409ddad99d27e7a177ed6b1da37e85a45bc9a98d.jpg

http://www.paul-revere-heritage.com/one-if-by-land-two-if-by-sea.html