The Boston Massacre was an incident in Boston, Massachusetts in 1770 in which 5 American colonists were killed by British soldiers. It was hugely important as a subject of propaganda for the Americans who were pushing for more independence from Britain.
Since late 1768, British soldiers had been in Boston to keep the peace. Soldiers in those days were not very well-disciplined and were often likely to treat civilians poorly. They were also poorly-paid and competed with civilians for jobs that they tried to work while not on duty. Finally, the civilians were also required to house and feed the soldiers. All of this meant that there was a lot of hatred between the two sides.
In March of 1770, an incident erupted between soldiers and civilians. Civilians started doing things like throwing rocks, ice, and snow at the soldiers. The soldiers shot into the crowd. Three civilians were killed outright and two died later of their wounds.
The “massacre” was very important because of how well the American patriots used it. They portrayed it as an unprovoked massacre of innocent civilians. This helped turn many American colonists against the British. Thus, the massacre was an important incident that helped lead more Americans to desire independence from Britain.