'Paul' (probably not Paul) is writing to his faithful followers in Colossae, who he does not know personally, but about whom he has been hearing good things from his friend Epaphras.
Paul has been struggling for the Colossian congregation and another one at Laodicea, both of whom have been tempted by false preachers. He informs them that they have been circumcised, not by a Mohel but by Jesus, and not actually but spiritually. Also, when you get baptised you die and then you're reborn. Another thing Jesus did: nail the Old Testament laws to the cross. So no one can tell you what to eat or drink or when to party like it's 62AD. In fact, you don't have to follow any earthly rules at all! Rules are for fools and don't actually help you to keep your base nature in check.
Sex and death. Death and sex. Since the world is ending and Jesus is coming back, we need to put our best feet forward, which means no more fornicating, nay, not even lusting after people, greed or evil concupiscence (v. 5), whatever that is. Also, no anger, blasphemy, swearing or lying, which will be hard, considering we lie 10 times a day. Anyway, trade all that fun stuff in for meekness, humbleness, forgiveness, charity, because that's what Jesus would do.
All that jazz in the last chapter about the lack of need for rules? Well, that was only for the firstborn sons of firstborn sons. Wives need to submit to their husbands, husbands need only love their wives, children have to be obedient, and parents shouldn't provoke their children and finally, slaves need to obey their masters, even when they're out of the room.
Slave owners need to be just and fair to their slaves. What that means, or how that's possible with that kind of power dynamic in place is not explained. Try to be an interesting conversationalist so you can convert people.
Paul informs them that he's sending his associate Tychicus and Onesius to them and also sends regards from a bunch of other people and asks them to forward the letter to Laodicea when they've finished.