In verse 13, Paul tells the Thessalonians not to sorrow over those who have died in Christ like others who have no hope. Here in verse 14 he tells us of that hope. Those who know The Lord continue to live with Him after the death of their mortal bodies, and they will return with Jesus on the day of the Rapture (versus 15 and following).
Lord, you are the giver of hope; Father You. We may sorrow for lost loved ones who knew you because we miss them, but we do not sorrow as the world that has no real hope. We know that we shall see our brothers and sisters in Christ again, and we rejoice for them that they are now in a happier state.