Okay, digital data is supposed to be easy 1 and 0 communication. But when you get down to the physical media, said binary digits are represented by physical phenomenon. So +3.3V = 1, 0V = 0 type stuff.
Voltage, resistance, EM waves, magnetics, etc... You're actually back in the world of Analogue, and here you have to worry about noise.
When you're moving data as fast as you can, or storing it as densely as you can, interference becomes more likely. For example, you'd think that +3V =1 and 0V = 0 would be easy, but when you're flipping the signal as fast as you can, you end up with the cable possibly acting like a transformer or capacitor. So the voltage might run a bit higher, a bit lower, a bit faster, a bit slower, etc...
Radio transmissions, Solar noise, close by electrical cables, other data cables with parallel runs, etc... The world is 'noisy' even if you're using wires.
That's why you have error correction in digital communications. So the 'occasional' bit can become flipped and the system transparently recovers it, and you get your transmitted data, identical from the other side.