Their diagram is shown below.
I have added the connection from Opamp inverting input to Opamp output as this was shown by the D1 net labels but easily missed due to the pathetic diagram. quality. There was no need to use net-labels to sho this connection in this case, and doing so hides the classic unity gain buffer configuration.
When 100% of an opamp's output is fed back to the inverting input, as is done here, the output tracks the noninverting input. The output can drive whatever the opamp is capable of driving, whereas the input can be of low drive capability, needing only to be able to drive the opamp input.
The opamp noninverting input "sees" the voltage at the R_LDR & R1 common point =
Vin = Vcc x (R1/(R1 + R_LDR)
An important point, which they appear to have missed, is that the LM358 opamp has a maximum allowable input voltage of less than Vcc by as much as 1.5V at 25 C or as much as 2V across the whole temperature range.
This means that at 25C when Vcc = 5V, the maximum input voltage that the IC can deal with is 5 - 1.5 = 3.5 VDC. If the input voltage is ever higher than 3.5 VDC with Vcc = 5V then the output may be indeterminate.
A look at their picture shows R1 = 10k.
As above, the voltage into the opamp = Vcc x (R1/(R1 + R_LDR)
This will equal 3.5V when 3.5V drop across R1 and 1.5V drop across R_LDR. so this occurs when R_LDR = 1.5/3.5 x 10k = 4300 Ohms.
As the LDR resistance drops with increasing light, the upper legal light limit is when R_LDR = 4200 Ohms, BUT the LDR is shown on their Wiki page as decreasing to as little as 1K at 100 lux. (There is shown to be a spread from 1k to 2k for typical product).
The light value where Vin = 3.5V can be read from the graph. As can be see, when LDR = 4k3, lux level = somewhere in the 40 to 70 lux range. As the LDR is shown as being 1K at 100 lux, someopamps will allow less than half the desired range to be measured. In practice many opamps may exceed 3.5V common mode ranmge and measurable lux level will be higher.
Maximum lux level is shown as 100 lux. That is a level which is adequate for reading but well below what is recommended for domestic illumination. Full sunlight is 100,000 lux and a typical overcast but not totally stormy day may be 10,000 lux. So the 100 lux limit of the sensor seems very low for interesting experimental purposes. The PCBA is an OK price at $5 (although someone like Sparkfun would be expected to sell something this simple for much less) BUT in many cases, buying an LDR and adding a resistor and feeding 5V in, with no opamp buffer, would produce an equally useful result, plus the ability to select an LDR liable to be more generally useful.