What is meant by "begotten" in "Jesus was begotten before the worlds"?

The [Athanasian Creed](https://www.newadvent.org/cathen/02033b.htm) , printed in copies of the 1662 Anglican Book of Common Prayer, declares that our Lord Jesus Christ is >"God, of the Substance of the Father, begotten before the worlds; and Man, of the substance of his Mother, born in the world. Perfect God and Perfect Man; of a reasonable soul and human flesh subsisting." What is meant by "begotten" in this quotation?