What is "New Creation" according to Catholicism? 2 Corinthians 5:17 (ESV)
  
  Therefore, if anyone is in Christ, he is a new creation. The old
  has passed away; behold, the new has come.


I don't know if this subject has importance in Catholicism. Therefore, I want to know what Catholicism has to offer regarding this topic.

How has Catholicism explained what is "New Creation"?
Well, as a Catholic, I'm not going to attempt to interpret the Bible, here's the commentary from the New American Bible:


  A new creation: rabbis used this expression to describe the effect of the entrance of a proselyte or convert into Judaism or of the remission of sins on the Day of Atonement. The new order created in Christ is the new covenant.


I asked a similar question concerning St. Thomas Aquinas' Hymn  concerning the what he meant by the New Rite and apparently he's talking about the New Covenant and the New Covenant is the Gospel message which is Salvation through Christ. 



At a superficial level, I'd assume he was talking about Baptism.  When you're baptized you're made a "new creation" in Jesus.  The new creation would be the new life of the spirit dwelling within the newly minted Christian.