Heaven is a spiritual realm ruled by God and populated by angels and the souls of the deceased, righteous mortals. Lucifer used to reside in Heaven before he was condemned to rule Hell for eternity by his father.
Not much is known about Heaven's history, other than it was created by God as a dwelling place for him, his angels and, later on, the souls of the deceased good people. It is usually associated with the upwards direction, the opposite of Hell.
When a righteous soul reached Heaven, they would be greeted by Uriel. He would welcome them with a speech that Lucifer described as "worse than Hell".