Basically a 555 works as follows:
When the voltage on pin 2 is less than 1/3 of the IC's supply voltage (Vcc), the RS FF is set and the output (pin 3) goes high (almost up to Vcc).
When the voltage on pin 6 is more than 2/3rds of the supply voltage, the RS FF is reset and the output pin goes low (grounded to the negative supply line).
A pot is connected across the battery to be monitored with the wiper to pin 2 and adjusted so that the voltage on pin 2 equals 1/3 Vcc when the battery is at the desired low voltage disconnect point. When the battery voltage goes below this setpoint, the output voltage on pin 3 will go high.