It’s a biblical reference. The “prodigal son” took his father’s inheritance, wasted it and came home humbled, but accepted by his father anyway, to the consternation of his brother who stayed around taking care of their father. It can be used fairly literally when someone returns home from an extended absence. It can also be used sarcastically.
Edit: Sarcastically in that many people identify with the put-upon brother who feels resentment towards the “prodigal son” who returns home and basically gets rewarded for being an idiot. The moral is supposed to be that being jealous and petty like the brother is a bad look. You should be happy that your brother is back in your life, as the father is. In my opinion, it’s one of the toughest parables in the Bible because it’s just *so* against human nature.
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