From Fr. Z's "What Does the Prayer Really Say" blog, the Pope will be celebrating Holy Mass today ad orientem using the main altar of the Sistine Chapel during which he will baptize 13 children.
Full story from Fr. Z.
The Pope celebrating Holy Mass ad orientem using the main altar of the Sistine Chapel means that he is celebrating Holy Mass facing towards God or versus Deum per Iesum Christum instead of the people or versus populum. In simplest terms, the Pope and all the people assisting (the entire congregation) at Holy Mass will be facing towards one direction literally.