It means that the person has no desire to sin whatsoever.
pure of heart” means never saying or thinking or doing anything that would hurt someone or hurt yourself.
when it is purified and enlightened it becomes inspired. Just as intellect is the instrument of knowledge, heart is the instrument of inspiration. If we are pure we will reach GOD. “Blessed are the pure in heart, for they shall see GOD” A pure heart sees beyond the intellect.