Digital logic treats a range of physical voltages as logical low and another range as logical high. This abstraction lets us reason using zero and one without tracking every electron. The voltage region between guaranteed low and high input limits is not a guaranteed valid logic level.
Worked example
Imagine a device whose data sheet guarantees low below 0.8 V and high above 2.0 V. An input of 0.4 V is low and 2.5 V is high. An input of 1.2 V is in the unspecified region; assigning it a reliable Boolean value would be unsafe.
What to watch for
These example thresholds are illustrative, not universal. Supply voltage, logic family, loading and the particular device determine the actual limits.
Try it yourself
With those illustrative limits, is 1.8 V guaranteed high?
Answer: No. It is below the guaranteed high threshold and above the guaranteed low threshold, so the data sheet does not guarantee its interpretation.
How to study this topic
Begin with settled Boolean values, then consider the physical signal and the device's electrical limits. A logic symbol explains a function; its data sheet explains which voltages, currents and delays are permitted.
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Digital Electronics