Malachi 4:2…”But for you who revere My name, the Sun of Righteousness will rise with healing in its rays.” By the Sun of Righteousness we understand this to be Jesus Christ. Through Him believers are justified and sanctified, and are brought to see light. His influences render us holy, joyful, and fruitful. It is also a reference to the graces and comforts of the Holy Spirit, brought into our souls. Christ gave the Spirit to those who are His, to shine in their hearts, and to be a Comforter to them, a Sun and a Shield.
In terms of perfect, physical healing (which is not the main point of the passage, but is often used out of context in association with this verse), this will be carried out in heaven, but not in this life. Otherwise, there would be no sickness and death for Christians. Can God heal the sick, and does He do so miraculously? Absolutely, and it happens every day all over the world, but every one of those people will eventually die. Should we pray for physical healing? Yes, we should, with the expectation that God is big and powerful and sovereign enough to heal miraculously. What if I prayed for healing, but the person wasn’t healed; does that mean I didn’t have enough faith? No, there is not a guarantee of physical healing in this life, and all humans are still affected by the curse of sin, which includes sickness and death. Does He heal non-Christians? Yes, which shows that He is not a cosmic genie, dependent upon our prayers and/or faith for healing to occur.