A 741 op amp is employed as a voltage comparator. The noninverting input is connected to zener reference source. Reference voltage is 5V. R2 is adjusted so that the voltage at the inverting input is half the supply voltage. When supply is higher than 10V, the LED will not light. When the supply falls just fractionally below the 10V level, the IC inverting input will be slightly negative of the noninverting input, and the output will swing fully positive
The LED will light, indicating that the supply voltage has fallen to the preset threshold level. The LED can be made to light at other voltages by adjusting R2.