1. Old North Church was tied into the Sons of Liberty. “One if by land, two if by sea”. If the British Regulars were heading to Concord via land (around the Boston neck, Roxbury, etc.), they’d put one lantern in the cupola. If they were coming by sea, across the Back Bay, they’d put two lanterns in the cupola.
2. Paul Revere, Richard Dawes (and some other guy?) go off to warn the countryside that the British Regulars are on their way. This news travels fast and by early morning, militia had massed on Lexington Green in an attempt to prevent the Regulars from advancing.