As a prophet he passionately declares that the King is coming, and that anybody who wants to be incorporated into His Kingdom needs to first be converted, this means: cleansed of heart by confession and repentance. People who confess their sins with the intention to have their lives renewed are then baptized by John in the Jordan River, as a sign of their cleansing.
When Jesus is around thirty years of age, He comes to the Jordan to be baptized by John. Initially John refuses; he knows that Jesus needs neither conversion nor cleansing. Nevertheless, Jesus’ intention is to accomplish all commandments and statutes of God, and so to ‘fulfill all righteousness’.
Galations 4 verse 4.
For our sake He made Him to be sin who knew no sin.
2 Corinthians 5 verse 21.
So this baptism was a demonstration of humility by Jesus.
After the baptism, the Holy Spirit descends on Jesus in the form of a dove, and the voice of God the Father speaks:
"This is my beloved Son, with whom I am well pleased."
The anointing with the Holy Spirit equipped the Messiah for His public mission as the 'Servant of the Lord'.
Jesus had been circumcised eight days after His birth.
Luke 2 verse 21.
Being baptized by John, Jesus has now brought his 'circumcision' to fulfillment.
The circumcision was the sign of the covenant with God.
This covenant is being continued and came to its fulfilled form in the
‘Kingdom of God’.
Like for instance the Old-Testament-Pesach came to its fulfilled form in the Last Supper, so the 'circumcision'. being the covenant-sign, has come to its fulfilled form in the ‘Kingdom-baptism'
Psalm 2 verse 7 and 8:
I will tell of the decree:
The LORD said to me, “You are my Son;
today I have begotten you.
Ask of me, and I will make the nations your heritage,
and the ends of the earth your possession".
Isaiah 42 verse 1:
"Behold My servant, whom I uphold,
My chosen, in Whom my soul delights;
I have put My Spirit upon Him;
He will bring forth justice to the nations".
Matthew 17 verse 5:
He was still speaking when, behold, a bright cloud overshadowed them, and a voice from the cloud said:
“This is my beloved Son, with whom I am well pleased; listen to Him".
Colossians 1 verse 13 and 14: He has delivered us from the domain of darkness and transferred us to the kingdom of His beloved Son,
in Whom we have redemption, the forgiveness of sins.
John the Baptist was the forerunner or herald of King Jesus. His mission was to prepare the people for His arrival.