The word “we” here refers to all Christians, including Paul’s readers. (See: rc://en/ta/man/translate/figs-inclusive)
This is a metaphor, comparing the control of the “principles of the world” to a master-slave relationship. AT: “the customs and laws of our ancestors controlled our behavior” (See: rc://en/ta/man/translate/figs-metaphor)
Possible meanings are 1) this refers to the laws or moral principles of the world, or 2) this refers to spiritual powers, which some people thought control what happens on earth.
Paul uses the metaphor of a person buying back lost property or buying the freedom of a slave as a picture of Jesus paying for his people’s sins by dying on the cross. (See: rc://en/ta/man/translate/figs-metaphor)
This is an important title for Jesus, the Son of God. (See: rc://en/ta/man/translate/guidelines-sonofgodprinciples)