What does Acts 2:39 mean?

Peter has just called a crowd of Jews to repent and be baptized. Some of them must have recognized the words of John the Baptist who had his ministry on the Jordan River just east of Jerusalem three years prior. If so, they may remember John's harsh words to the Pharisees and Sadducees who believed they were saved by virtue of being descendants of Abraham. John told them don't rely on pedigree. "God is able from these stones to raise up children for Abraham. Even now the axe is laid to the root of the trees. Every tree therefore that does not bear good fruit is cut down and thrown into the fire" (Matthew 3:9–10).

Peter knows the apostles are to be Jesus' witnesses "in Jerusalem and in all Judea and Samaria, and to the end of the earth" (Acts 1:8), but he doesn't yet know that this is more than a geographic description. His audience is of devout Jews and proselytes (Acts 2:5, 11), all dedicated to Judaism. It won't be for a while that the apostles recognize that when Jesus said He had "other sheep that are not of this fold" (John 10:16) He meant the wretched Samaritans (Acts 8:14–17) and the blasphemous Gentiles (Acts 10:44–48), as the Jews would have seen them.

Jesus' offer of salvation was always meant to be offered to the Jews first, but, equally, it was always meant to be offered to the Gentiles as well (Romans 1:16).