This circuit gives an output (which in this case is OV) when an input voltage lies in between two specified voltages. When it is outside this window, the output is positive. The two op amps are used as voltage comparators. When Vin is more positive than Vref (upper) the output of ICl is positive and D1 is forward biased. Otherwise the output is negative, D1 reverse biased and hence Vout is OV. Similarly, when Vin is more negative than Vref (lower), the output of IC2 is positive; D2 is forward biased and this Vout is positive
Otherwise Vout is OV. When Vin lies within the window set by the reference voltages, Vout is OV.