What's the red dot above the signal bars?

It should be an ORANGE dot and that indicates that your microphone is in use by an app. The other color is a GREEN dot which indicates that your camera is in use. These are user awareness features which alert the user that either their microphone and/or the camera is in use.

Axel F.

The red dot is an orange dot that is displayed as red with certain background colors to make it stand out better.

See the following from Status icons and symbols on your iPhone - Apple Support

With iOS 14, 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.

A green indicator  means either the camera or the camera and the microphone are being used by an app on your iPhone.

If you’re talking about an orange dot, it means an application is using the microphone.