A missionary is a person whose mission is to go somewhere to help others. In many cases, the goal of a missionary is to teach about a religion so that the people convert to that faith.
Missionary can be a noun — the person who goes on a mission — or an adjective — the type of work done on such a trip. If your great-grandmother was a missionary for the Methodist church in Japan in the 1920’s, her purpose there was to convince people to abandon their religion for Methodism. Presumably she did this missionary work because she thought it would save their souls.