2 John is a short letter, in the Bible (New Testament). The author was John, who also wrote John's Gospel, Revelation, and two other letters.
The book of 2 John teaches that we should love other Christians. God orders us to love each other (2 John 5-6).
We must not accept people who teach wrong things about Jesus (2 John 7-11). Jesus is God. But he became a man so that he could die for us. When he died, he suffered the punishment for our evil deeds. This is the true Christian message.