Sometimes you hear people ask, what do you mean? And other times, the same people seemingly ask the same question in a different format-changing the “what” to “how”-and you get lost.
The big question is are they( those two sentences) synonymous or not? Well, they are not synonymous; hence they are used in different contexts.
When someone asks what do you mean, it implies he totally did not understand what was said or understood little and hence needs a complete explanation.
How do you mean, on the other hand, implies the person understood what was said but needs some more clarification on it.
Consider these two scenarios:
1. Dennis: Martin, the government of this country is stupid?
Martin: Stupid? What do you mean?( by that) [ He did not understand how a government can be termed stupid]
2. Dennis: Martin, the government of this country is very corrupt?
Martin: Hmm, how do you mean?( in which way) [ He understood the statement but wants his friend to explain why he thinks the government is corrupt]
So when you do not understand a statement made, what do you mean is used, but in context where you only need a clarification, use how do you mean.