The given circuit is an OP-AMP based battery charge monitor.
Circuit constantly watches the power supply voltage and if the supply voltage goes "below" a certain voltage (6V in our case) it will indicate by glowing the LED.
Circuit is constructed using OP-AMP under comparator mode. Non-inverting terminal of the amplifier is biased to reference voltage(3V) using a zener diode. The non inverting terminal is connected to a voltage divider that devides input voltage equally.
If the input voltage drops below 6V then, voltage divider output drops below 3v and hence triggers the output of OP-AMP to go HIGH.
