Herod had earlier arrested John and had him chained and put in prison. He did this because John had told him, "It isn't right for you to take Herodias, the wife of your brother Philip."
Herod had earlier married Herodias, the wife of his brother Philip. But John had told him, "It isn't right for you to take your brother's wife!" So, in order to please Herodias, Herod arrested John and put him in prison.