It means wherever you’ve located this sensor, it receives a variable amount of illumination at night. Some nights it might 36 lux, 55 lux on other evenings, and probably other values as well.
Simply leaving the threshold at 50 probably won’t cover all the possibilities (as was demonstrated the previous night). I still recommend you raise the threshold to 60. In addition, you may wish to use the Min/Max sensor to gain insight into the sensor’s lux levels over many days.