What is the button for in 3.1?

Please help me understand a button/sign - "MINUS SIGN INSIDE a CIRCLE" - that appears in my 3.1 A (AT@T) Notification strip (after swiping from above) between the battery and flashlight buttons.

Sorry for a stupid question, but I am not experienced in cells and the 3.1 manual is extremely meager and poor.

Thank you very much for assistance and help.