Paul's second (preserved) letter to the church in Corinth.
Paul is generally held to have written this letter shortly after his departure from Ephesus during his 3rd missionary journey, in about AD 55 or 56. Paul had apparently written a "severe" letter previously and wanted to make things right with the church in Corinth.
Paul had sent fellow laborers to Corinth prior to this letter and was planning to visit Corinth in the coming months himself (which he did). Paul had spent extensive time in Corinth during his second missionary journey and knew this church well.
Paul's surviving correspondence with the Corinthians indicates that one or more additional letters were written by Paul to Corinth that have not been preserved.