The book of Philemon is a short, personal letter. It is in the New Testament part of the Bible.
Paul wrote some encouraging words to Philemon. Then, Paul appealed to Philemon on behalf of Philemon's slave. The slave was called Onesimus. Onesimus had escaped from Philemon. Later, Onesimus became a Christian. He helped Paul. But Philemon still owned Onesimus, because Onesimus was a slave.
Paul decided to return Onesimus to Philemon. Paul asked Philemon to accept Onesimus as his Christian brother, instead of a slave.