Learn English – Is “What does it mean by [something]?” a correct utterance

It doesn't mean it's an important thing; in fact it's sort of the opposite. What Ronan means is that he wished they learned more than "a bit", but he's glad they're at least learning something. He might have said instead "Well, that's better than nothing."

This is generally used when you wish something more/better than what is happening were possible, but are accepting what you can get. For example:

Jim: "Did you get everything we needed for the party? Sally really wanted those special crackers she likes."

Mary: "They didn't have those, but I did find paper plates with unicorns on them!"

Jim: "Well, that's something. Sally does like unicorns."

Wherein Jim really wishes Mary had gotten the crackers, but is pleased that at least something Sally likes will be present.

The sentence might mean what you intended, but the usage is very strange. Ordinarily, a person is dumbstruck by a specific event, such as a revelation of information, and the reason for being dumbstruck is considered important enough to be mentioned immediately - probably in the same sentence.

Having asked you a question, one would expect that he would not speak until you answered his question, so silence on his part is expected. He might be dumbstruck by your answer, but not before you gave it. So I don't think that "dumbstruck" is an appropriate word under the circumstances.