1 Thessalonians
This is one of the earliest letters that Paul wrote. He didn’t have much time to spend with these new believers before he was run out of town by persecutors (Acts 17). So he writes this letter to answer some questions that have come up. One of the main concerns is about whether Jesus is coming back or not. Paul explains this in chapters 4-5. This is one of the only places where it clearly explains what the “rapture” is.
2 Thessalonians
Shortly after the first letter, Paul writes them a second, shorter letter. Apparently the church is really being persecuted, just like Paul was when he was with them (ch. 1). Someone also is spreading a nasty rumor that Jesus had already come back and they missed it. So Paul explains how they can know this is absolutely false (ch. 2). As he usually does, he gives them some general encouragement to do things that please the Lord.