The Liquid Contact Indicator turns red when excessive moisture is detected inside the iPhone. While this isn't covered under the standard warranty, AppleCare+ will cover it under accidental damage after payment of a small fee.
In some mobile sets red sensor shows the charged battery status. If your set is charged then sensor will show green color, otherwise it will show red color.
In some mobile sets red sensor shows the charged battery status. If your set is charged then sensor will show green color, otherwise it will show red color.