What does the word circumcision of the heart mean?

This refers to The Bible, speaking of the old covenant between the Jews and the Abrahamic god, in which circumcision was required as a sign of reverence and adoration. However, for Christians who accept the New Testament, this no longer required due to the new covenant that was made with Jesus. Circumcision of the heart thus became a metaphor for someone who was pious, kind, or devout.