Paul begins his letter to the Colossians by calling himself an apostle, in Greek apostolos. According to Vatican II, all Christians, even the laity, are called to an “urgent and many-sided apostolate,” a mission to live and spread the Gospel wherever we go and in whatever we do (1). We are all apostles.
But what is an apostle? We can get a better idea of the meaning of this word (and our mission) by studying the Greek word apostolos. The noun is derived from the verb apostellō, which means “to commission, send forth” (2). The verb, in turn, is comprised of two parts: the preposition apo, meaning “from, away from” and the verb stellō, meaning “send” or “dispatch.” Stellō, however, has another meaning: to make ready, arrange, equip, and set in order.
Based on the simple, primary meaning of the verb apostellō, then, we usually define an apostle as someone who is commissioned or sent forth, and this is true. God does send us out into the world to serve our fellow human beings in His name and to live and spread the Gospel, just as Vatican II says.
But if we stop there, we miss an important piece of the puzzle. God doesn't just send us out; He makes us ready first. He sets things in order for the success of our mission. He arranges what we are to do. And He equips us for doing it. That's what the stellō part of the verb tells us. God doesn't just leave us to go off on our own, using our own meager resources and powers. He first provides us with everything we need, and He continues to supply us during the whole of our apostolate.
So when Paul calls himself an apostle, he certainly means that he was sent by God, but he also indicates that he is sourced by God, Who makes him ready for his mission, equips him with everything he needs, and paves the way for his success. His job is to cooperate and rely on God's support every step of the way. The same is true for us, apostles that we are, as we, too, live the Christian life.
(1) Decree on the Apostolate of Lay People, paragraph 1 (Flannery edition).
(2) Information on Greek words and meanings comes from Bible Hub and Perseus Digital Library.