“It's strictly confidential.”

You work at a doctor's office. At work, you have access to a lot of your patients' personal information. You're explaining your job to an extended family member who doesn't know much about it, and you want to explain that you can't share any information about the patients. When you're talking about the private information, you say:

It's strictly confidential.

(something) is strictly confidential

If something is "strictly confidential", it means that you absolutely can't tell anyone.

The word "strictly" means "completely" or "absolutely". "Confidential" means "secret", in the sense that you're not allowed to tell anyone.

Information that is described as "confidential" is:

• Military information or secrets
• Patients' medical information
• Conversations that you have with your lawyer

"Confidential" is an adjective, so you can use it to describe ideas:

I'm not allowed to share any confidential information.

You said this was supposed to be a confidential conversation.

"Strictly" isn't used very much. It mostly fits into a few set phrases, including the one above and:

Strictly speaking, we're not actually married.

Smoking is strictly prohibited.

I'm sorry to do this to you. It's strictly business, you know.