This circuit comparator uses one voltage as a reference (pair of two resistors act as a voltage divider) and is going to be the reference voltage of the op-amp 2.5V but can be a variety of sources like a photo-diode to detect when there is light or not. What it does is when the voltage at V+ is lower than the reference voltage, the difference of the value is going to be negative(Vi+ - Vi-) and the op-amp there is going to be 0 V at the Vo, which can simulate a logic value of low, and when the V+ is higher then the reference voltage there is going to be 5 V at Vo, simulating a logic value of high.
There is something on the circuit that can't be seen which is the rail voltages of the op-amp, in this case, the voltage of the negative rail should be connected to the ground but it was applied 0 V to be the low value that the op-amp can achieve on Vo.
The positive rail of the op-amp is set for 5 V which is going to be the higher value on the Vo.
So every time the voltage on V+ changes above or under 2.5 V (Reference voltage) there is going to be a change of value in the Vo.
This can be seen as an interface between an analog and a digital bit converter.
