St. Thomas Aquinas
“The actions performed by the priest in mass are not ridiculous gestures, since they are done so as to represent something else. The priest in extending his arms signifies the outstretching of Christ’s arms upon the cross. He also lifts up his hands as he prays, to point out that his prayer is directed to God for the people, according to Lam. 3:41: ‘Let us lift up our hearts with our hands to the Lord in the heavens’, and Ex. 17:11: ‘And when Moses lifted up his hands Israel overcame.’ That at times he joins his hands, and bows down, praying earnestly and humbly, denotes the humility and obedience of Christ, out of which He suffered.”
Source: St. Thomas Aquinas, Summa Theologica