Jesus answered him, “Truly, truly, I say to you, unless one is born again  he cannot see the kingdom of God.” 4 Nicodemus said to him, “How can a man be born when he is old? Can he enter a second time into his mother's womb and be born?” 5 Jesus answered, “Truly, truly, I say to you, unless one is born of water and the Spirit, he cannot enter the kingdom of God. 6 That which is born of the flesh is flesh, and that which is born of the Spirit is spirit.  7 Do not marvel that I said to you, ‘You  must be born again.’ 8 The wind  blows where it wishes, and you hear its sound, but you do not know where it comes from or where it goes. So it is with everyone who is born of the Spirit.” (John 3:3-8 (ESV)
“Unless one is born of water and the Spirit, he cannot enter the kingdom of God.” Jesus talks of faith as entering the kingdom of God, or seeking the kingdom of God. This is impossible to do or have without the work of the Holy Spirit upon us. It is impossible to believe apart from the work of God. He has to intervene, he has to take our hearts of stone and replace them with hearts of flesh. Natural man, he cannot believe in God, he is bound to sin. It is God who works faith, it is God who works repentance. He does this through his word, and he does this through baptism. That is what he is talking about here. Baptism. Jesus is referencing Ezekiel 36:25 ff
“I will sprinkle clean water on you, and you shall be clean from all your uncleannesses, and from all your idols I will cleanse you. 26 And I will give you a new heart, and a new spirit I will put within you. And I will remove the heart of stone from your flesh and give you a heart of flesh. 27 And I will put my Spirit within you, and cause you to walk in my statutes and be careful to obey my rules.” (Ezekiel 36:25-27 (ESV)
God sprinkles clean water on us, and with it the Spirit is poured upon us, giving and confirming faith.