The Heidelberg Catechism

What is the meaning of these words "He was conceived by the Holy Ghost, born of the virgin Mary"?

That God's eternal Son, who is, and continues [112] true and eternal God, [113] took upon him the very nature of man, of the flesh and blood of the virgin Mary, [114] by the operation of the Holy Ghost; [115] that he might also be the true seed of David, [116] like unto his brethren in all things, [117] sin excepted. [118]