High level and low level are mainly used in digital circuits. It is reflected in the circuit that there are only "yes" and "no", and there is no intermediate value. The voltage value of this "yes" is different in different circuits.
If it is a digital circuit powered by 5V, the high level is 5V, or close to 5V.
Low level is‘ none ‘, which is 0V or close to 0V.
The meaning can be described as switching on the lamp. Turning on the lamp means closing the switch. 220V voltage is directly added to the bulb, which means high level to make it glow. When the lamp is turned off and the switch is disconnected, the voltage on the bulb disappears, that is, the low level.
The difference is the voltage used in the analog circuit. This voltage is continuous and has the current value.
For example, when you turn on the power supply, the light bulb lights up differently according to the position of the dimming knob. It has any value in the middle except the brightest and darkest.
If the change is not too fast, you can use a multimeter to measure it directly. Generally speaking, no matter what level (such as CMOS, TTL), the high-level voltage should be greater than 3V and the low-level voltage should be less than 0.8V, which is generally the potential of GND. If the level changes quickly, you can‘t measure it with a multimeter. Use an oscilloscope.
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