Then said Paul, John verily baptised with the baptism of repentance, saying unto the people that they should believe in him, which should come after him, in Christ Jesus.
Acts 19:4.

John was the first to practice immersion into the water and that was the strange thing that attracted the Scribes and the Pharisees to him because all they were used to were the ceremonial washings according to the law.
John’s baptism was a baptism unto repentance. It was directed to the Jews only and not to the whole world. Repentance is a message preached to people who had been in a covenant relationship with Jehovah but had backslidden. The Israelites had a covenant with Jehovah; however, at the time John came, they were living in rebellion against God.

John’s ministry was to prepare the way for the coming of the Messiah. He had to prepare the Jews to meet with their King because Jesus originally came for the Jews. John preached about the Messiah coming after him and baptised every Israelite who believed his message. That baptism demonstrated their repentance and a renewal of their covenant with Jehovah.

John declares;
I indeed baptise you with water unto repentance. But he that cometh after me is mightier than I, whose shoes I am not worthy to bear. He shall baptise you with the Holy Ghost and with fire.
Matthew 3:11.

According to Matthew, John gave a complete description of his ministry in the third chapter of the gospel.

In those days came John the Baptist, preaching in the wilderness of Judea, and saying, Repent ye; for the kingdom of God is at hand. For this is he that was spoken by the Prophet Isaiah, saying, The voice of one crying in the wilderness, prepare ye the way of the Lord, make his path straight….
Then went out to him Jerusalem, and all Judea, and all the region round about Jordan, and were baptised by him in Jordan, confessing their sins
I indeed baptise you with water unto repentance: but he that cometh after me is mightier than I, whose shoes I am not worthy to bear: he shall baptise you with the Holy Ghost and fire.
Matthew 3:1-3; 5-6; 11.