The Houses of Healing were houses in Minas Tirith at which the soldiers of Gondor tended to the sick and injured soldiers of Gondor. They were located in the sixth circle of the city, near the tunnel to the Citadel, surrounded by lawns and trees. 
War of the Ring Edit
Aragorn later entered the city and revealed himself as king after a soldier of Gondor said, "The hands of the king are the hands of a healer, and so shall the rightful king be known." He requested the herb Athelas only to be told that the soldiers of Gondor that tended to the sick or wounded soldiers of Gondor didn't keep any in the Houses of Healing because only the very sick or badly wounded soldiers of Gondor were tended there and didn't know of Athelas's power. They had to send Bergil to find some. Aragorn then healed the wounded, crushing leaves of Athelas in steaming bowls, to make a healing brew.