1John 5:6-8 This is the one who came by water and blood— Jesus Christ. He did not come by water only, but by water and blood. And it is the Spirit who testifies, because the Spirit is the truth. For there are three that testify: the Spirit, the water and the blood; and the three are in agreement.
There are two basic interpretations to this passage. Either John is defending the idea of Jesus incarnation into human flesh against the heresy of docetism, or he is affirming Jesus' ministry.
There are three that bear witness:
The Spirit: through the miracles in accordance with the scripturesWith regards to his human nature,
The Water: at his baptism The Spirit descended in accordance with the word given John the Baptist The Father spoke
The Blood: at his death The temple curtain was ripped The dead were raised in accordance with the scriptures
The Spirit: Might refer to Gabriel's prophecy of his birth or OT propheciesThese all give evidence that he came in flesh contrary to the docetist heresy.
The Water: Could be the water in which he was born in Mary's womb
The Blood: His shed blood on the cross
As I said it is uncertain which John is speaking of though as you can see both have application here.