As was said above, what passes through the wire that we CALL a digital signal is really an ANALOG signal. It is subject to distortions. This video is a pretty good, clear and simple explanation of what can go wrong:
https://www.youtube.com/watch?v=grzoqEb2KMk&feature=youtu.be
https://www.youtube.com/watch?v=grzoqEb2KMk&feature=youtu.be