It's a phrase referring to several historical incidents, particularly the start of the American Revolutionary War in 1775. The phrase comes from Ralph Waldo Emerson's "Concord Hymn". Historically, there was no single shot that could be blamed to be the start of the war since shots were fired earlier at Lexington, Massachusetts where eight Americans were killed and a British soldier was wounded, and it was described more as a massacre rather than a battle. The reason why eight Americans died that night was because British troops had been ordered to seize weapons and ammunition that the colonists had stocked up on. It's uncertain which side shot first, but it was definite that the Revolutionary War had started after this occurrence.
