Verse By Verse Through Galatians (1:18)

18 Only after three years did I go up to Jerusalem to confer with Cephas, and I stayed with him fifteen days. -Galatians 1:18 BSB

After three years of Paul’s conversion to Christianity—having an encounter with Jesus Christ, after His resurrection and ascension to heaven, see Acts 9:1-6, he visited Peter.

Paul stayed with Peter for several days, as he was one of the pillars of the faith, see Galatians 2:9, most likely getting more insights about Jesus and what He taught while He was on earth with His disciples so that Paul would be much more informed and equipped for preaching the gospel.