All varieties of n are nasal! The difference, in Welsh, between n and nh is that the former is "voiced" (the vocal cords vibrate) and the latter is "voiceless" (the vocal cords do not vibrate: it's, if you like, a "whispered n").
That's not a distinction that many languages make -- but if you really want more information (...than you probably need!) see the Wikipedia article on the "Voiceless alveolar nasal".
However, the practical reality is that mae nhw in the mouths of nearly all speakers sounds so like "maenw" (IPA ['mainu]) (pronounced -- as ibisc goes on to say -- exactly as if it were one word) that the difference is not really worth bothering about. Just say "maenw"! :)