A threshold is the voltage at the boundary between “0 (Low)” and “1 (High)” in mainly digital circuits.

When a voltage higher than the threshold is applied to the IC input terminal, it is recognized as being “High.” When the voltage is lower than the threshold, it is recognized as being “Low.”

When a voltage close to the threshold value is applied, the state becomes unstable without it being known whether it will be “High” or “Low” due to IC variance, fluctuation in the applied voltage, etc., which could cause the IC to malfunction.

For this reason, the “High voltage” and “Low voltage” specifications for the IC input terminal are generally set to voltages that are a distance from the threshold value.