What is the meaning of "adultery of the heart," what is its recompense, and how can one get rid of its whispers?

Thinking about the good qualities of a foreign man is forbidden, based on the Prophet's (peace be upon him) saying: "The son of Adam has his share of adultery decreed for him, which he will inevitably attain. The adultery of the eyes is looking, the adultery of the ears is listening, the adultery of the tongue is speaking, the adultery of the hand is touching, the adultery of the foot is walking, and the heart yearns and desires, and the private parts either confirm or deny that." As for thinking about one's husband, there is no harm in it.