With digital images, dark tones are represented with very few values. With a digital sensor, there is measurement error called noise. The digital sensor noise is always of low numeric values— hence the errors (noise) show up in the dark tones. Noise increases and creeps up into the mid-tones with higher ISO (as the sensor readout is more amplified).
As a simple example, a pixel that should read as pure black (a value of zero) might actually measure at 0, 1, 5, 8 13 or 20 (or whatever). A pixel that should measure at 17 might read out at 7, 13, 17, 23 or whatever. This error is random by pixel, but in practice there can be streaks/banding and/or grid-like patterns (particularly ugly). Colored speckles and a color cast are typical, as well as brightness variation. It gets worse at higher ISO values.
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