This letter was written to the church in Thessalonians, in northern Greece. Some theologians think that this is Paul's earliest letter and therefore the oldest book in the New Testament. Lots of people became Christians when Paul visited Thessalonica, but many more people were unhappy about this, so they kicked Paul out of the city. This meant that there were lots of fresh but immature Christians left behind, who needed guidance. Paul wrote this letter in response to questions that the believers were asking about what will happen when Jesus returns, and what happens when we die. He also reminded them to live a holy life meanwhile.
This letter contains discussion about some of life's bigger questions. It reminds us that even though we have questions, we can be sure about what Jesus has done, and we can live for Jesus until he comes again. It's a great read for anyone who has recently become a Christian, as that's the audience it was written for.