The Great Commission in Christian belief is something that Jesus told his followers. This happened after his resurrection. Jesus told his Twelve Apostles to go all over the world and tell everyone about Him. The most well known version of the Great Commission is in Matthew 28:16 to 20. Jesus meets his followers on a mountain in Galilee. He tells them to make disciples of and baptize everyone in the name of the Father, Son and Holy Spirit.
Earlier Jesus had sent out 70 of his followers to preach and heal, but only to the Jews. However, Jesus himself had healed some non-Jewish people before. At the beginning of his public work, Jesus spoke to His own people. After rising from the dead, he made it clear that His message was for everyone. This "commission" is very important to missionaries who believe that God wants them to go to every country in the world and preach about Jesus.