The 228 Incident refers to the events of February 28, 1947. Japan returned Taiwan to the Republic of China in 1945 after its defeat in World War II, but after 50 years separated from Chinese rule, tensions existed between locals and the new government authorities.
The incident itself started on February 27, 1947, when the Tobacco Monopoly Bureau sent agents to confiscate illegal cigarettes from a street vendor. A conflict erupted in which one citizen died from a gunshot, leading to a full-scale uprising against the government. Protestors issued 32 demands to the government, including democratization.
The military put down the uprising with an iron fist, sometimes going as far as shooting anyone who broke curfew or breaking into homes to question the inhabitants.
It's impossible to tell how many people died as a result of the mobilization, but academics put the figure at somewhere between 10,000 and 30,000 people.
Today, February 28 is marked as Peace Memorial Day, a national holiday. There are a museum and a foundation in Taiwan that are entrusted with preserving the memory of the incident and its countless victims.