The difference would be the priest would face the altar with his back to the people (ad orientem) as opposed to facing the people over the altar (versus populum).
“Ad orientem” is Latin for “to the East”. It does not mean “his back to the people”. This is a common misconception.
The priest offers Mass facing the same direction as the people, because he and the people together are offering worship and sacrifice to God. He is not turning his back on the people to exclude them. Rather, as a Christian community, are all facing ad orientem (i.e. toward the east) waiting in joyful expectation for the coming of our Lord Jesus Christ who will return to judge the living and the dead and the world by fire (Rite of Baptism, 1962).
What in the early Church determined the position of the altar was that it faced Eastward. To quote St. Augustine: “When we rise to pray, we turn East, where heaven begins. And we do this not because God is there, as if He had moved away from the other directions on earth…, but rather to help us remember to turn our mind towards a higher order, that is, to God.”
This quotation shows that the Christians of those early days, after listening to the homily, would rise for the prayer which followed, and turn towards the East. St. Augustine always refers to this turning to the East in prayer at the end of his homilies, using a set formula, Conversi ad Dominum (“turn to face the Lord”).
The priest faces east (to the Lord) because he is offering the Mass in Christ’s name and in His Person, in persona Christi, to God the Father and is leading his people in adoration and worship. He is facing east, the rising sun, which is symbolic of the ‘New Jerusalem’ and he is leading his flock as the Good Shepherd does.
My parish only has a high altar, and all Masses are offered ad orientem. The opening prayers, liturgy of the word, and closing prayers are not done at the high altar. They are done at the ambo or the reading stand while facing the people.