This poem, as its subtitle reveals, was written to commemorate a stone monument erected in 1837 to celebrate the Battle of Concord, an important event in the American Revolutionary War. In other words, "Concord Hymn" itself is a commemoration of a commemoration! This will be a poem about history, memory, and heroism.
The first stanza describes the Battle of Concord itself—a 1775 battle which, alongside the Battle of Lexington, kicked off the American Revolution. Lines 1-2 set the scene, describing how the "embattled farmers"—a rough-and-ready amateur army—placed a flag in the ground as an act of defiance. It's not certain what flag these revolutionaries "unfurled," but it certainly wasn't the British one! The battle took place near a crudely built bridge over a river, here described dramatically as a "flood."
It was on this very site—where the speaker now stands with a crowd—that the "embattled farmers stood," both literally and in the sense of standing their ground. The words "Here once" stress that this was one specific moment in the powerful flow of time, subtly setting up the idea that, unless people make a serious effort to remember them, the fighters' heroic deeds will be easily swept away and forgotten.
Line 4 uses hyperbole to describe the bullet the farmers fired at the British colonialist soldiers as the "shot heard round the world." Though it's not known for certain which side fired first, this one line has been so influential that many people assume it to be factual. The point is that this first shot sets in motion a chain of events that effectively ends colonial rule and gives birth to the U.S. as a free, independent, and powerful nation. This shot is not literally "heard round the world," but its consequences do change the entire world.
It's no exaggeration to say that this one line is by far Emerson's most famous. It has since been used to describe the assassination of Archduke Franz Ferdinand—which sparked the First World War—and the slaying of President John F. Kennedy in 1963. The muted /d/ consonance in the line—"fired the shot heard round the world"—makes the sharp /sh/ and /t/ of "shot" jump out like a sudden explosion.