What it means
Nadi sorts each person into Adi, Madhya or Antya by their Moon nakshatra, and the classical reading ties it to health constitution and progeny. When both partners fall in the same one the koot scores zero, and that zero is what people mean by Nadi dosha.
How it is read
It carries more weight than any other koot, so a pair can look fine on the other seven and still land under the conventional 18-point threshold. There is no partial state on the way there — Nadi rarely scores in between.
What to watch for
Two cancellations are well established and both are common: the same nakshatra with different padas, and Moon signs in 1, 5 or 9 from each other. A Nadi dosha reported without checking them is a partial reading. Note what is not a cancellation — same Nadi with different nakshatras is the textbook pattern, not an exemption.