Digital

Digital is using something analog in a weird way.
Normally we use absolute values, e.g. a cartridge of a turntable sending a signal to a phono stage.
In case of digital we still use analog signals (there are no digital electrons) but instead of using the absolute value, we are looking for a marked change of state.
A transistor is open or closed, a magnetic field is N-S or S-N, a pit or a land (remember CDs?), etc.
All the time there is very marked change of state.
Conceptually this is represented by our bits, a binary unit that can have only 2 values also conceptually represented by 0 and 1.
This is what makes digital so robust. We don’t use the absolute values, we use this marked change of stet to detects out bits.

Digital signal transmission

This can done sending block pulses over a wire.
In case of audio, SPDIF, USB or Ethernet are well known examples.

Pulse
This is how a block pulse might looks in real life.
If this would be used as an analog signal (using the absolute value), this would be a disaster.
The over/under shoot, the rise/fall time, the ringing, all would manifest themselves as distortion.
The moment our interpretation is digital (the signal remains analog of course), we don’t have these problems.
Despite all the distortions, we can clearly detect the rise to approximately 100% and the fall to approximately 0% of the signal.
As long as we are able to do so, the receiver will detect the bits properly.
This is why digital  is to a large extend insensitive to noise.