What does this red circle on my iPhone means?

Expnewphone wrote:

Yes. Keeps showing up. Pls don’t reply to thread unless you know what it is or how to fix.

This forum has some of the most knowledgeable people in the world when it comes to Apple troubleshooting. However, no one here is a mind reader. Unless we ask questions, we don't know what troubleshooting steps you have already taken. We also may need more information. Be prepared to answer clarifying questions or be prepared not to get much help.

Best of luck.

There is an answer from a community specicalist on this --> Red circle on screen and noise - Apple Community

Or if you were referring to this icon as shown below

Then see the pic below which is self-explanatory

So did you follow the suggestion in that thread and force restart your phone?

Please show the screenshot of your iPhone. That may give more clarity