There is a kind of culture myth about "Paul Revere's Ride." This event really occurred and it was really important, but it has been mythologized and romanticized. Paul Revere did embark on a long horseback ride on April 18th, 1775, to warn people along the route, so they could organize a defense, just as NealC has said.
In the myth, Paul Revere cried out "To arms! To arms! the British are coming!"
However, this is impossible on the face of it. He couldn't possibly have called them "the British." In 1775, everyone in what is now the United States was British! (Of course). We considered ourselves British. Only later did the British become "them" and not "us."
He might have yelled "To arms! To arms! The Redcoats are coming," because at that time the British army wore bright red uniforms... while the American colonists had no uniforms at all.
I don't want to over-stress this, but the idea of ordinary citizens owning rifles for hunting--and having available to defend themselves against military invasion--is part of what plays into US "gun culture" and support for gun ownership. The "Minutemen," the citizen-soldiers poised to be armed and ready at a minute's notice, are US culture heroes.
Paul Revere's Ride was immortalized by the poet Henry Wadsworth Longfellow, in a poem which most of us learn in school, beginning
"Listen, my children, and you will hear
Of the midnight ride of Paul Revere.
'Twas the eighteenth of April in 'seventy-five;
Hardly a man is now alive
Who remembers that famous day and year..."
As children, we thought it was hysterically funny to chant
"Listen, my children, and you shall hear
Of the midnight attack of diarrhea..."
It refers to the Battle of Lexington and Concord, which took place in 1775. It was the opening battle in the American Revolution from Britain.
The British sent troops to arrest some American leaders and confiscate some arms hidden in Concord. Paul Revere and some others rode ahead of the British troops, warning American Militia that the "British Were Coming". A small group met the British for a few moments at the Lexington Green, the British dispersed them and marched to Concord. By the time they got there the whole countryside was ringing church bells to rouse up American Militiamen to resist the British.
The British return march through the militia opposition resulted in many casualties and was nearly a disaster.
So I guess the bells reminded the character of these battles, most people educated in American schools would know the reference.