Loras has been kept in a cell for the majority of season six of Game of Thrones because the Faith of the Seven wants to punish him for having sex with a man. Although his sister Margaery has been able to pretend to be repentant in order to be free, he is not able to do the same. In the epic season finale, he makes an interesting plea: when the High Sparrow asks Loras whether he is able to stand trial, Loras replies that there will be no trial. He confesses all of his sins right there in front of everyone. He then declares that he wishes to devote his life to the Seven. Pleased with this, the Sparrow obeys his wishes. This leads to a brutal scene in which Loras gets a familiar symbol carved into his forehead.
But what does this symbol mean? It's actually the Seven Pointed Star, which signifies the unity of the seven aspects of the deity worshiped by the Faith of the Seven. We have seen many of the followers have it on their foreheads throughout the season. These aspects are: The Father, who represents justice; The Mother, who represents mercy, childbirth, and peace; The Maiden, who represents purity, love, innocence, and beauty; The Crone, who represents wisdom and foresight; The Warrior, who represents courage in battle; The Smith, who represents creation and craftsmanship; and lastly The Stranger, who represents death and the unknown. These are all the faces of the seven-faced God, and the symbol is most likely meant to be a daily reminder to those who devoted their life to the God.