This is the meaning of repentance: a turning of the direction of our life and the affections of our heart, so that we become oriented on God and love the things he loves. John promises the people “forgiveness of sins” in response to their repentance, their turning to God, but he calls them to demonstrate the seriousness of their turning by accepting baptism in the Jordan.
This was a remarkable demand of John on his Jewish kinsmen. In the context in which John lived baptism had one main significance among the Jews: it was the symbolic rite that proselytes had to go through to become Jewish. This made John’s baptism very offensive. It implied that unless the Jews were willing to repent, they were not really Jews and could not count on the promised blessings God had made to his chosen people. Or to put it another way, in calling Jews to accept a baptism of repentance for the forgiveness of sins, John was telling them that they cannot rely on their Jewishness for salvation; they have to be changed in their heart toward God.
And Luke’s understanding of John’s baptism is that it implied that the way was open for Gentiles to repent and be forgiven. If Jewishness does not save, then Gentilishness does not necessarily condemn: the issue is repentance toward God.