Mission - Webster's Dictionary definition
noun:
A mission literally means a something that is sent (from the Latin missum(sent).
An operation that is assigned by a higher headquarters; " the planes were on a bombing mission".
A task that has been assigned to a person or group; "a confidential mission to London"; "his charge was deliver a message".
What is that something assigned by a higher headquarters and assigned as a task to a person or group?
Jesus told His followers that "As the Father has sent me, so I send you - the Father and I are one".
The higher headquarters is God Himself who sent Jesus into the world and sends us into the world. The task that has been assigned to us as individuals and as a group (Christians) is to tell all who will listen that God loves them, that He sent His Son to live, die, and then live again, conquering death so that payment could be made for our sins, and we could be reunited with God who created us to be with Him.