In the Camera App, the phone detects parallel lines and a horizontal or vertical level will appear to assist you in taking pictures. When guidelines are white, the image is not level. The lines will turn green when you have adjusted your position to level the picture.
When holding your mobile phone vertically:
When holding your mobile phone parallel to the ground:
The Horizontal Level is set to on by default. If you don't want it to show, open the Camera App > Settings (in the top-left corner) > Set the Horizontal level toggle to the off position.
