What does “the ear of the heart” mean?

What did Christ mean by the words: “He who has ears to hear, let him hear” (Matt. 11:15)? St. Jerome comments on this phrase as follows: “Isaiah says, ‘The Lord has assigned an ear to me’ (Isa. 50:4). Let us understand what he says. The Lord has assigned an ear to me, that is the ear of the heart; He has granted me an ear that hears message of God. Whatever the prophet hears, he hears in his heart. Thus, when we also cry in our hearts, ‘Abba Father!’ with a silent cry, the Lord hears this silence in the same manner as the Lord converses with our hearts that cry, ‘Abba Father’ (Rom. 8:15).”