Ad orientem are the Latin words for, “toward the East.” In Catholic usage it can mean either the geographical east or the east of any Catholic Church building regardless of the actual geographical orientation of the building. Thus, when entering any Catholic Church, the area where the altar is located is the “eastern” side of the Church.
From the earliest centuries of the Church until about 1966, Catholic Mass was celebrated with all, bishop or priest facing the altar in the same direction as the laity, facing the east, facing Jerusalem, facing the rising sun, facing the direction from whence Jesus will return at the end of time and also at the altar at the consecration. It is a mistake to say that the priest has his back to the people, as true as this might look, but it is more accurate to say the priest is facing the same direction as the laity and in a leadership stance or marching forward to the Kingdom of God as on a pilgrimage.
Ad orientem makes clear what is spoken by the priest is directed toward God when facing the same direction as the laity. And it makes clear too, what is spoken to the laity by the priest, when he turns toward them for greetings, etc.
It also takes the emphasis off the priest’s looks, piety and personality. The priest “stands in” for Christ at the altar, especially at the consecration. Facing the same direction as the laity, the priest could be anyone, the pope or the lowliest priest. It doesn’t matter.
Ad orientem also emphasizes the Sacrificial aspect of the Eucharistic Prayer. It is not a reenactment of Holy Thursday’s Last Supper. It is a prayer to God recalling how Jesus memorialized His Good Friday Sacrifice on the night before He died. The words, “take eat” and “take drink” are not meant for the congregation in the Eucharistic Prayer. It is directed to God recalling Jesus’s words. The Bread and Wine become the Risen Lord and His one Sacrifice is offered to the Father though the priest representing Christ during the consecration and afterwards. At the Sacrificial Banquet following the Canon of the Mass is when God gives back to us His Crucified and Risen Son.
The Eucharistic Sacrificial Banquet or “meal” occurs after the Lord’s Prayer when the priest turns to the Congregation and says, “Behold the Lamb of God…, blessed are those called to the Supper of the Lamb". The Rite of Holy Communion is the “Sacrificial Meal” aspect of the Mass, not the Eucharistic Prayer. This is confused when the priest faces the congregation during the entire Eucharistic Prayer and makes it appear to be words directed to the Congregation, which of course it isn’t.This is especially true if the celebrant motions to the people with the bread and chalice as though the words are directed to them as apostles at the Last Supper. Of course, the Mass is not a memorial of the Last Supper, the Mass, established at the Last Supper is the Memorial of Good Friday and Christ's One Sacrifice on the Cross, but in a glorified way, due to the Resurrection of our Lord on Easter Sunday.