Information is digital if its range consists of a number of discrete values. Any value not in the discrete set is forbidden. Clearly the so-called natural numbers (i.e., the positive integers) are digital in nature, as are the binary integers (0 and 1), or bits. Other examples of things that might be considered digital include the abacus, our DNA, shoe sizes. Even the alphabet has a discrete number of symbols in it, and could be considered digital. Indeed, in your computer, each symbol on your keyboard is actually stored, manipulated, and transmitted as a pattern of binary digits, or bits.
Transmission systems can also be digital. When they are metallic (copper), they can use discrete voltage levels to represent digital information. For example, the presence of a 3 volt signal could be interpreted as a one (1) and the absence of any signal as a zero (0). Or a +3 volt signal could be a one (1), and a -3 volt signal a zero (0). If we use multiple voltage levels, we can represent multiple digital values with one signal. For example if the receiver can distinguish between +6 volts, +3 volts, -3 volts, and -6 volts, then we could assign the bit pattern 00 to +6 volts, 10 to -6 volts, 01 to +3 volts, and 11 to -3 volts. Now with a single signal, we would be sending two bits to the receiver. Optical systems can likewise use the presence or absence of light to represent digital information.
One of the benefits of a digital transmission system is noise reduction. Every signal attenuates as it travels, and picks up noise and distortions. If we need to go a great distance, uniformly amplifying a signal (which is what must be done in analog transmission systems) uniformly amplifies the noise and distortion along with the original signal. Over great distances this can produce significantly degraded quality. In a digital system, however, we can regenerate or repeat instead of amplifying. As long as the repeater or regenerator can correctly read the digital information from the transmission system, it can then create a brand new, clean signal on the next transmission segment, eliminating the accumulated noise and distortion as it does so.