An altar is an object used in religious worship rituals. In ancient biblical Judaism, a golden altar was used in the Temple for burning incense unto God while a bronze altar was used for burning sacrifices and offerings for the atonement of people's sins. In modern religious use, an altar is simply a piece of furniture whereupon those who feel God's call on their lives would lay their lives down as a sacrifice.
In the Left Behind series, an altar was used in the Holy of Holies in the Temple that appeared during the Tribulation for the Jews to worship God. During the midpoint of the Tribulation, Nicolae Carpathia desecrated the Temple by sacrificing a pig in the Holy of Holies and splashing its blood upon the altar, enraging the Jews and forcing him to break the seven-year agreement that he had made with Israel.