This is going to be one of a few books of the Bible where we only have one chapter and so this is our only opportunity to speak about the book and speak about the content of the book. Over history people have assumed that this is about a runaway slave, Philemon, who happens to be caught and is in the same prison as Paul. Paul brings him to a saving knowledge of Christ and sends him back to his master, Onesimus. Paul encourages his master to treat him as a brother and that if he is owed any money that Paul would repay him.
I’m not so sure, and since it is not a sure thing, I’m allowed to come up with my own theory, which is based on Scripture, of course. I see Philemon and Onesimus as being blood and religious brothers. They are literal brothers and brothers in the faith. Philemon has stolen something from Onesimus and so was arrested. Paul encourages Onesimus to forgive him and to receive him back and welcome him as if he were welcoming Paul.
Paul does use the word slave to describe Philemon, but he also uses the word slave to describe himself in various times in Scripture. Look at Romans 1:1-15 where we read about his self-description as a slave. This is not uncommon. When Jesus washes the feet of the disciples he is exemplifying what it means to be a servant. He tells his disciples to be servants. The term is not unusual and it does not need to be taken literally. This is especially true when over history it has been taken literally and it has served as fodder to support the treatment of those who have been blessed by the sun more than others.