The 2 diodes on the left clamp the signal so you don't explode your op amp. The op amp is set up to be a comparator which outputs +15V and -15V. On the right side of the op amp there is a zener resistor and 2 zeners to clamp the signal to +5.7V and in the end another diode so the output is 0V~5V TTL. This circuit is a cheap way to get a clock signal direct from the power grid.
