The preceding quotation from Solomon's Proverbs receives further explanation.
59. Perchance you will ask how I came to cite, as referring to the Incarnation of Christ, the place, “The Lord created Me,” seeing that the creation of the universe took place before the Incarnation of Christ? But consider that the use of holy Scripture is to speak of things to come as though already past, and to make intimation of the union of two natures, Godhead and Manhood, in Christ, lest any should deny either His Godhead or His Manhood.
60. In Isaiah, for example, you may read: “A Child is born unto us, and a Son is given unto us;” so here also [in the Proverbs] the prophet sets forth first the creation of the flesh, and joined thereto the declaration of the Godhead, that you might know that Christ is not two, but One, being both begotten of the Father before the worlds, and in the last times created of the Virgin. And thus the meaning is: I, Who am begotten before the worlds, am He Who was created of mortal woman, created for a set purpose.
61. Again, immediately before the declaration, “The Lord created Me,” He says, “I will tell of the things which are from eternity,” and before saying, “He begot,” He premised, “In the beginning, before He made the earth, before all hills.” In its extent, the preposition “before” reaches back into the past without end or limit, and so “Before Abraham was, I am,” clearly need not mean “after Adam,” just as “before the Morning Star” need not mean “after the angels.” But when He said “before,” He intended, not that He was included in any one's existence, but that all things are included in His, for thus it is the custom of Holy Writ to show the eternity of God. Finally, in another passage you may read: “Before the mountains were brought forth, or ever the earth and the world were made, You are from everlasting to everlasting.”
62. Before all created things, then, is the Son begotten; within all and for the good of all is He made; begotten of the Father, above the Law, brought forth of Mary, under the Law.
Source: On the Christian Faith (De fide) (New Advent)