What does it mean if revere saw one lantern in the belfry?

In "Paul Revere's Ride" by Henry Wadsworth Longfellow, seeing one lantern in the belfry signifies that the British troops are approaching by land. This is part of the warning system established by Revere and his fellow patriots to alert the colonial militias. The lanterns serve as a signal: one if by land, two if by sea, indicating the route of the impending British attack. This imagery underscores the urgency and importance of the message Revere is tasked to deliver.