You can read the text here.
Through Timothy, Paul had heard that the Thessalonians had some anxiety about those in their congregation who had died before Jesus returned. Paul issues the Thessalonians some reassurance. There is no disadvantage to those who died prior to Jesus return. They will join with Jesus when he returns just like those who are still alive will. They will not miss out on the resurrection. Jesus himself confirms this. Grief is ok, but not grief without hope. The language Paul uses is perhaps a bit odd, talking of meeting Jesus in the air. At the core it's an expression of the fact that Jesus is not mere human king. His kingdom includes and transcends that.
Paul moves on in the next section to give them further comfort via eschatology. His goal is to reinforce what they already know because it is critical and to further comfort them. Paul affirms, Jesus is coming back some day, and it will happen suddenly. It also won't be a pleasant return for everyone as some will experience his wrath. The Thessalonians, however, have nothing to fear. They are God's children and live in a manner worthy of their status. Paul encourages them to continue to live out their identity and to live...'in light of the certainty and unexpectedness of that Day's coming.' It will be a day of their vindication, a day that will begin a blessed eternity living under the rule of the divine King rather than the rulers of this world. In the meantime they will be protected in part by their lives of faithful love for God and one another and their resolute expectation in Jesus' salvific activity on their behalf. Critically, it is a message that the Thessalonians are to remind each other of, that way they can persevere even when the going is tough.
 This concern makes most sense to me if 1 Thessalonians is very early. If it had been 15-20 years since Jesus ascension, then this concern would be odd since surely many Christians has died before his return. Of course this causes problems with Acts' narrative, but I agree with Campbell that Paul's own letters deserve priority when dating the letters. Campbell's date (argued for on other grounds) between 40-42 is plausible.
 I don't want to make too much of this, but it's interesting that Paul doesn't say that they're in heaven with God after they die.
 Drawn from Gaventa.
 Fee is very helpful in drawing out the theme of comfort in 5:1-11.
 If only Paul was more explicit here about who will receive wrath. At minimum it appears to be their Roman overlords and those who wholeheartedly invest their system. Fee suggests it's those who are making life difficult for the Thessalonians, which is true probably true as well.
 Malherbe p. 289.