You simply say, "I agreed to keep my employer's business confidential when I went to work for them. It's important to me to be trustworthy."
If you are firm and polite about this, you will identify yourself as someone who is business-savvy.
Of course, it's perfectly reasonable for an employer to ask "What are you doing in your current job? What frameworks and tools are you familiar with? What problems have you solved lately? What would you do differently if you could?
In most cases you should be able to answer questions like that without disclosing your employer's secrets. For example, if you're in software and you're developing a cross-platform mobile app, you should have plenty to say about how that is done. You can talk about your development, test, and deployment process. You can talk about the standard tools you use. If your employer has custom-built modules in the tool chain you can say "I use a custom tool."
If they ask, "what's the release schedule, who is your product for, and what's the price?" you should say, "I'm sorry, I'm sure you understand it's not my place to talk about that."
Rarely, companies invite people for sham interviews to try to obtain inside information from competitors. But they usually do that by talking to executives and product managers.