These famous words were spoken by Colonel William Prescott, the military commander in charge of patriot forces at the Battle of Bunker Hill. His order passed into legend almost as soon as it was made, and has been endlessly quoted in a variety of contexts, both military and non-military, ever since.
Colonel Prescott’s order served a very practical military purpose. He didn’t want the American militia under his command to use up precious ammunition on the enemy unless they were certain it could be used to maximum effect. In telling his men not to shoot until they could see the whites of the enemy’s eyes, then, he was effectively urging them to bide their time until the moment was ripe for them to open fire.
Even so, the American forces defending Bunker Hill eventually ran out of ammunition anyway—they even resorted to throwing rocks and muskets at one point—but not before they’d inflicted severe casualties on the British. Though the Battle of Bunker Hill has gone down in history as a British victory, it was largely a Pyrrhic victory in that it left British forces in a much weaker state, and demonstrated conclusively that the Americans had shown they were prepared to fight fiercely, and would not be easily defeated.