Grid lines are very helpful to take better shots with your iPhone and is disabled by default on the iOS. To turn on and enable camera grid on iPhone, do as follows:
1. Open the Settings application on your iPhone.
2. Scroll down to the Photos & Camera option and tap to open the settings
3. Under the Camera settings, make sure to toggle on the Grid button
4. Now, get back to the camera app and you should see the lines on the screen.
5. To turn grid off on the iPhone's camera, simply toggle the Grid button OFF.