The question is a reference to the two witnesses of Revelation 11.  To understand the passage we must understand the context.  This is figurative, as is most of the book.  Really to understand this question we must look to the purpose of the Revelation.  It would be difficult to narrow the meaning of one sentence, but if we had to I think that we could adequately narrow the theme, or the point of the book, to the fact that God will overcome Satan, indeed He already has (Genesis 3:15).  The figures used are thus implemented to demonstrate the victory, that is to represent the struggle that the faithful go through in their spiritual war against Satan (Ephesians 6:12).
    In verse 3 the two witnesses are introduced.  Some have tried to attach an identity to them, but I believe that the number represents unity or confirmation (in the Old Testament two witnesses were needed for a conviction, and in the New Testament Jesus sent out His disciples in groups of two).  These witnesses prophesied 1260 days, or three and a half years.  This is simply an indefinite period of time (especially as opposed with its double, 7, which means completion or perfection).  They were mourning because the holy city had been trampled.  Even in this period of chaos and destruction their minds were fixed upon God (as should be the minds of all faithful Christians today).
    Down in verses 7,8 they are killed by the beast (representative then of the world opposing the righteous teaching of the church).  In verse 9 their bodies are to be seen three and a half days (an indefinite period which is much shorter than that in which they preached).  In verse 10 the evil worldly people gloat over their supposed victory (evil glories at any blow taken by right).
    In verse 11 the two were brought back to life.  In verse 12 they are shown as being rewarded and victorious.  Verse 13 shows that 7000 of the evil men (symbolic of complete [7] destruction) would be slain, thus showing the victory of the church over the world.
    Victory has been won already!  When Jesus hung on the cross Satan thought he had succeeded, but in actuality when Christ arose from the grave He crushed the head of the gloating serpent.  The victory has been gained, but the faithful will not taste of that victory until, in judgement, the righteous are rewarded and the wicked punished.