Ephesians 4:7-10

But grace was given to each one of us according to the measure of Christ's gift.  Therefore it says,
“When he ascended on high he led a host of captives,
    and he gave gifts to men.”
(In saying, “He ascended,” what does it mean but that he had also descended into the lower regions, the earth? He who descended is the one who also ascended far above all the heavens, that he might fill all things.)

Ephesians 4:7-10

Paul has told his readers about unity and oneness with regards to God, faith, baptism, etc.  But, Paul wants to be clear that not everything can be applied to this "oneness" idea.  He points out that there are gifts given that are different for each person.  To back up his point, Paul refers to Psalm 68:18.

Now, before Paul continues his thought on these gifts, he pauses to talk about how the verse in Psalms says "he ascended".  Paul tells his readers that the very fact that he ascended means that "he also descended".  This verse is referring to Jesus, who descended to earth.  He descended and he ascended.  Not only did he ascend, but he "ascended far above all the heavens".  The purpose of this is "that he might fill all things".  This "filling" reminds me of some earlier passages.  As I did a quick search to refresh my memory, I noticed the end of chapter 1.  Christ was raised from the dead and seated at the right hand of the Father.  His seating was far above all things, which sounds similar to what Paul is pointing out about how he "ascended far above all the heavens" in Psalm 68.  Ephesians chapter 1 ends with the fact that Christ "fills all in all", or "fills all things everywhere with himself" (NLT), or "fills and completes all things in all [believers]" (AMP).  Christ is the one who fills and completes all things with himself.  It is for that purpose that he descended and ascended.