Indochina was a French colony east of Thailand and south of China with the South China Sea to the east, so named due its position halfway between India and China. In the mid 20th century, the colony birthed three countries: Cambodia, Laos, and Vietnam.
During World War II, Indochina was under the nominal rule of the Vichy government of France, which gave Japan permission to base troops in Indochina. Admittedly, if France had denied this to Japan, the Japanese would have gone into Indochina anyway. This way, France maintained a ghostly sort of sovereignty over the area.
Following France's defeat in the Great War, the Empire of Japan "persuaded" France to hand over the former French Indochina for a suitable amount of compensation which came as a respite for the battered French economy.