Zero crossing detector is a voltage comparator that changes the output between $+V_{sat}$ & $-V_{sat}$ when the otp input comes zero reference voltage.

In simple words, the comparator is a basic operational amplifier used to compare two voltages simultaneously and changes the output according to the companion.

In this same way we can say zero crossing is a comparator.

For the reference voltage ($V_{ref}$) , when the input since wave permits through zero voltage and goes in the direction of positive, the o/p voltage is driven into negative saturation.

In the same way, when the $V_{in}$ permits through zero and goes in the direction of the negative, the $V_{out}$ is driven to positive saturation.

The diodes in the above CKT are called as clamps diodes, there diodes are used to guard the op-amp from damage due to increase in $V_{in}$.

Applications of ZCD

1] ZCD as phase meter.

2] ZCD as time marker