The phrase "main altar" refers to the primary or most important altar in a religious or sacred space, such as a church or temple. It is the central place where religious ceremonies or rituals take place. Full definition
For example, it can not be decided by any theological reflection alone whether or not the tabernacle is best placed on the main altar. (religion-online.org)
People were standing on every available surface, including the main altar. (religion-online.org)
On Saturday, July 16, 1054, as afternoon prayers were about to begin, Cardinal Humbert, legate of Pope Leo IX, strode into the Cathedral of Hagia Sophia, right up to the main altar, and placed on it a parchment that declared the Patriarch of Constantinople, Michael Cerularius, to be excommunicated. (christianitytoday.com)