Those are units of ADU (A/D Units). CMOS sensors are analog devices. Photons are converted to electrical charges, and the charges that are collected by each pixel are digitized by analog-to-digital (A/D) converter(s) on the sensor into numerical values. The ADU represents the numerical value of the charge and thus is a representation of the amount of photons that are received by said pixel.
In the above, each ADU value is between 0 and 65535 (largest 16 bit number).
Max is the largest (maximum) value among all pixels in the sensor. Min is lowest (minimum) value. Ave is the mean value ("average") over all pixels, and STD is the standard deviation of the ADU from the mean.
https://en.wikipedia.org/wiki/Standard_deviation
The histogram is a curve that is an approximation of the probability density function for all pixels.
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