Why is there a red line flashing around the screen border randomly? [SGS6]

[AC Question]

I can't think of any settings I've changed, but all of the sudden, today a red line randomly flashes around the border of the screen of my Galaxy s6. How can I stop it?

 

[B. Diddy]

Welcome to Android Central! Go to Settings>Accessibility, and see if any of those features are turned on. Also check Settings>Developer Options.