أَوْ مَاهِيَّةُ الْمُسَمَّى أَيِ الْحَقِيقَةُ الَّتِي هُوَ بِهَا هُوَ
Or mā asks for the essence of what is named: the reality by virtue of which that thing is what it is.
Once an object and its existence are known, a question like mā huwa may seek its essential definition rather than a merely accidental description. A passing feature may not adequately answer a question about what the thing essentially is.
Exposition of a name clarifies the meaning of the expression; inquiry into essence seeks the reality of the referent. The context determines which kind of knowledge mā requests.