What do we find out in the exposition?

The speaker, a landlord telling stories with friends at the Wayside Inn in Sudbury, Massachusetts, prepares to tell the tale of Paul Revere's ride on April 18th, 1775. He begins with Revere telling his friend to light one lantern in the belfry of the Old North Church in Boston if the British are coming by land, and two lanterns if they are coming by sea. He will wait and watch on the opposite shore and then ride to warn the countryside. Revere departs as the moon rises over the bay, illuminating the British ship The Somerset.