“Hmm. That's a good question.”
You've given a presentation, and now you're answering questions about it. One of the audience members asks you a question that's tough for you to answer. You say this.
Hmm. That's a good question.
This is a noise that English speakers make to show that they're thinking hard about something. When you're trying to figure something out, make this noise:
Hmm. I'm not sure about that. Let me get back to you.
People say this when someone asks a question that's a little difficult to answer. Saying "That's a good question" gives you a little more time to think of your response.
For example, if someone asks you a question at work that you haven't thought about before and don't have an answer for, respond this way:
That's a good question. Let me find out for you.
Of course, you can also respond with "That's a good question" if someone asks a question that's really interesting and intelligent. But to be clear, it might be better to use a slightly different phrase like:
That's an excellent question!