1-3: John is addressing this letter to either a specific woman or a church. Though we can't know for sure, it doesn't affect the content of his message. John puts much emphasis in these opening remarks on "the truth" as well as Jesus' deity to counter the many deceivers spoken of in verse 7. Since Jesus referred to himself in John 14:6 as the truth, it makes sense that John would use the term synonymously with him.
4-6: John asks his reader to love her brothers through the obedience of God's commandments, just as he did in his first epistle. This commandment is not new because God required that we love our neighbors in the Old Testament.
7: It's important that we live in obedience to God's commands since there are so many deceivers in the world who reject that Jesus took on human flesh. John's emphasis on obedience was a reaction to the Gnostics who taught that the material world was evil so it doesn't matter how you live. Because matter was evil in their view, they also taught that God would never become a man.
8: Christians will one day be rewarded for the works done while in the body. We are saved by grace, but God kindly rewards us for our faithful service to him (1 Corinthians 3:14-15). Johns warning is for us to live in such a way as to not lose these rewards.
9: John gave his readers a clear way to discern who the false teachers are. Those who sway from the sum of Christ's teachings (meaning the Gospel) were never truly saved in the first place.
10-11: John warned his readers not to yoke themselves with anyone who brought a teaching contrary to the Gospel. In doing so, they would be supporting the false teacher's wicked works. Many Christians today want to partner with the cults in order to achieve a common good. But in doing so, they compromise the Gospel and their witness to the world.
12: John had a very personal relationship to this person or church and preferred to speak face to face regarding other topics he wanted to address.