The baldachin is at the centre of the crossing and directly under the dome of the basilica. Designed by the Italian artist Gian Lorenzo Bernini, it was intended to mark, in a monumental way, the place of Saint Peter's tomb underneath. Under its canopy is the high altar of the basilica. Commissioned by Pope Urban VIII, the work began in 1623 and ended in 1634. The baldachin acts as a visual focus within the basilica; it itself is a very large structure and forms a visual mediation between the enormous scale of the building and the human scale of the people officiating at the religious ceremonies at the papal altar beneath its canopy.

The form of the structure is an updating in Baroque style of the traditional ciborium or architectural pavilion found over the altars of many important churches, and ceremonial canopies used to frame the numinous or mark a sacred spot. Old St. Peter's Basilica had had a ciborium, like most major basilicas in Rome, and Bernini's predecessor, Carlo Maderno, had produced a design, also with twisted Solomonic columns, less than a decade before. It may more specifically allude to features drawn from the funerary catafalque and thus appropriate to Saint Peter, and from the traditional cloth canopy known as a baldacchino that was carried above the head of the pope on Holy Days and therefore related to the reigning pope as the successor of Saint Peter. The idea of the baldachin to mark Saint Peter's tomb was not Bernini's idea and there had been various columnar structures erected earlier.

The old basilica had had a screen in front of the altar, supported by 2nd century Solomonic columns that had been brought "from Greece" by Constantine I (and which are indeed of Greek marble). These were by the Middle Ages believed to have come from the Temple of Jerusalem and had given the rare classical Solomonic form of helical column both its name and considerable prestige for the most sacred of sites. Eight of the original twelve columns are now found in pairs halfway up the piers on either side of the baldachin.

According to wikipedia