Well, to start with, its meaning is noted in my bible, aside from that, it has been infallibly defined numerous times - the Council of Trent taught:
CANON II.-If any one saith, that true and natural water is not of necessity for baptism, and, on that account, wrests, to some sort of metaphor, those words of our Lord Jesus Christ; Unless a man be born again of water and the Holy Ghost; let him be anathema.Again, Trent teaches us: CANON V.-If any one saith, that baptism is free, that is, not necessary unto salvation; let him be anathema.