What does the 'bAt Lo' indicator on the display mean?

The 'bAt Lo' indicator flashes for one or two months before the batteries run out completely, warning you to replace them as soon as possible. If ignored, 'bAt Lo' will eventually stop flashing and remain on without flashing, indicating the thermostat and heating/cooling system have stopped working because the batteries are almost completely dead. After batteries are completely dead, the display goes entirely blank. (Pages 8–9)