Question

What do you think is meant by the expression the "shot heard 'round the world" ?

Answer 1

This phrase has become an idiom or a saying used to describe a event that had a great effect.   It originally was used to refer to the Revolutionary war.  The first shot in the USA fight against England.

Answer 2

When two countries where at war they faced each other at the battle of Gettysburg. The two countries decided to make a truce until a random gunshot was fired. At that moment, the two enemies started to kill each other. At the end of the battle, no one knows who fired the first shot.