What was the shot heard around the world and why was it important?

This is, of course, metaphorical. A shot is not literally going to be heard around the world, but it can set in motion events of global importance.

When the American Colonies declared it was time for England to give America her independence. England was not going to do it without a fight.

The American Revolutionary War, was the inspiration of other countries around the world, who wanted freedom and independence also.

When the guns were fired that began the first battle of the American Revolutionary War, it became known by the expression for "The shot heard around the world."