According to Catholicism, what does 'come upon thee' and 'overshadow thee' mean? When reading the verse regarding the miraculous birth of Jesus, there are certain terms used which I find very confusing.

How does the Catholic Church understand the bolded phrases in Luke 1:35


  And the angel answered and said unto her, The Holy Ghost shall come upon thee, and the power of the Highest shall overshadow thee: therefore also that holy thing which shall be born of thee shall be called the Son of God.


For context, Luke 1:30-36:


  And the angel said unto her, Fear not, Mary: for thou hast found
  favour with God.
  
  And, behold, thou shalt conceive in thy womb, and bring forth a son,
  and shalt call his name Jesus.
  
  He shall be great, and shall be called the Son of the Highest: and the
  Lord God shall give unto him the throne of his father David:
  
  And he shall reign over the house of Jacob for ever; and of his
  kingdom there shall be no end.
  
  Then said Mary unto the angel, How shall this be, seeing I know not a
  man?
  
  And the angel answered and said unto her, The Holy Ghost shall come
  upon thee, and the power of the Highest shall overshadow thee:
  therefore also that holy thing which shall be born of thee shall be
  called the Son of God.
  
  And, behold, thy cousin Elisabeth, she hath also conceived a son in
  her old age: and this is the sixth month with her, who was called
  barren.


If you say Jack will come up thee and over shadow thee it certainly brings certain thoughts to the mind, but how do Catholics understand this verse?
The phrases you point out indicate to the Catholic Church that the Holy Spirit was responsible for Jesus' conception:


  The Annunciation to Mary inaugurates "the fullness of time," the time of the fulfillment of God’s promises and preparations. Mary was invited to conceive him in whom the "whole fullness of deity" would dwell "bodily." The divine response to her question, "How can this be, since I know not man?" was given by the power of the Spirit: "The Holy Spirit will come upon you."


(Catechism of the Catholic Church, paragraph 484)

Or again:


  The Spirit comes upon the Virgin Mary and "overshadows" her, so that she might conceive and give birth to Jesus.


(Catechism, paragraph 697)

Aquinas, in his Catena Aurea, quotes Gregory of Nyssa on this passage:


  O blessed is that womb which because of the overflowing purity of the Virgin Mary has drawn to itself the gift of life! For in others scarcely indeed shall a pure soul obtain the presence of the Holy Spirit, but in her the flesh is made the receptacle of the Spirit. ... Moreover, the power of the Highest shall overshadow you. Christ is the power of the most high King, who by the coming of the Holy Spirit is formed in the Virgin. 


(emphasis added)

One sees the same thing in the Creeds:


  For us men and for our salvation
   he came down from heaven,
   and by the Holy Spirit was incarnate of the Virgin Mary
  
  (Nicene Creed)
  
  and in Jesus Christ, his only Son, our Lord,
  who was conceived by the Holy Spirit
  
  (Apostles' Creed)


(emphasis added)