Mā (ما) commonly asks for the nature or essence of a thing. Man (من) normally asks about a person or a rational being, seeking identity or class. These two interrogatives cannot be substituted indiscriminately.
«مَا الْحَرَكَةُ؟»
What is motion?
«مَنْ فِي الدَّارِ؟»
Who is in the house?
The first calls for an account of the concept or definition; the second calls for identification of an occupant. In particular contexts a question such as “mā Zayd?” can also ask about someone's kind or description. The kind of answer expected is an essential interpretive clue.