Enter the Bible has a very helpful introduction:
Paul begins this letter by blessing God. Such blessing is a common Jewish form of prayer. In this prayer, Paul gives thanks for God’s mercy and consolation. Suffering is also a theme as the letter opens.
Second Corinthians offers a real-life window on a strained relationship between a church leader and the people whom he loves. In this letter, we see an anxious apostle hoping to restore the relationship he had with churches he founded, even as he tries to avoid being drawn into a contest with other teachers over who has the most impressive skills and credentials. To do this, he discloses much about his own devotion to the Corinthians, his hardships in ministry, and the reconciliation God has accomplished for them all in Christ.