What does the red dot at the top of my iPhone 15 mean?

It may be orange but appear a bit red. One reason it would appear is that the microphone is in use when using the Phone (making or receiving phone calls). See the following from Status icons and symbols on your iPhone - Apple Support

With iOS 14 and later, you might see an orange or green indicator in the status bar on your iPhone. These indicators appear when the microphone and/or camera are being used by an app.

An orange indicator means the microphone is being used by an app on your iPhone.

Hard to say since we cannot see what you see.

Maybe the microphone activity symbol?

If not, and if it appears again maybe you can take a picture and post it here so we see what you see.

With that said, see the following support document for identification of status icons and symbols.

click here ➜ Status icons and symbols on your iPhone - Apple Support