مَا الْعَسْجَدُ؟
What is ʿasjad?
The speaker asks what the expression means; the answer is “gold.” This is an explanation of a name, i.e., a lexical explanation.
مَا الْإِنْسَانُ؟
What is a human being?
This may ask for the reality or essential characterization of the thing, classically described in logic as “a rational animal.” This is a question about the nature of the referent.
Not knowing the meaning of a word differs from not knowing what the named thing essentially is. The form mā may serve either purpose according to context.