What is the name of the app apple is talking about in this security warning?

Apple released security updates on February 13th, and you will want to apply those.

Here is the security notice: About the security content of iOS 16.3.1 and iPadOS 16.3.1 - Apple Support

The first of the two listed fixes is in the operating system kernel, while the second is in WebKit; in an HTML rendering library used by Safari and other tools.

What security warning exactly?

On February 13 Apple released an update. iOS 16.3.1, that fixes a couple of vulnerabilities.

It does not refer to any specific app. They are system fixes.

click here ➜ About the security content of iOS 16.3.1 and iPadOS 16.3.1 - Apple Support