We all know the story of the prodigal son who took his inheritance and lived a profligate life and finally went into penury. Then he returned to the father and asked forgiveness and his father whole heartedly embraced him. As an idiom it means a kind hearted father who forgives all the transgressions of his children.
My father is a prodigal son.
The heavenly father Christ is a prodigal son for all his sheep.