We find out at the beginning of "Paul Revere's Ride" that Revere has the task of waiting to see if the British army is coming by land or by sea. This will be communicated by lanterns placed in the church's belfry. The code is
One if by land, and two if by sea
Revere sees that it is two lanterns, so he jumps on his horse and gallops off to sound the alarm. As he does so, the iron shoe on his horse's hoof hits the cobblestones in such a way that the friction causes a spark. This is the origin of that "spark" in the quote that "kindled the land into flame."
In this passage, Longfellow takes a literal spark and turns it into a metaphor to describe the moment in history when the colonies turned to war with Great Britain. The spark of the "steed" is likened to the spark of energy that spread the flames of revolt among the Americans as Paul Revere galloped off to call the colonists to arm themselves against the British. Through this metaphor, Longfellow emphasizes that this was an exciting and dynamic moment in US history.
This is a deeply patriotic poem written in 1860, at a point of crisis and division before the Civil War. Longfellow is reminding Americans of their shared heritage in an emotional poem meant to unite them in memory of their common heritage.