Engineers who often use PLC are familiar with digital and analog IO, so what is digital?
According to Wikipedia, digital quantities are a kind of physical quantities, and their changes are discontinuous in time and always occur in a series of discrete moments.
It seems confusing, right? Let’s give an example to illustrate:
1, 2, 3, 4… Such signal changes are digital quantities, and 2, 3, and 4 after 1 are all integer multiples of 1, not 1, 1.1, and 1.111.

Going a little further, we can press the switch to turn on the light, and press the switch again to turn off the light. This is a digital quantity, and there are only two states: on and off.
If it is an electric light controlled by a knob, the brightness of the light can be controlled through the knob, it is an analog quantity, and it is a continuous change process.
Common digital IOs include PNP type, NPN type, relay type, counter type, PWM pulse type, etc.