The world is analogue. A measurement could be any value. We can turn these measurements into signals, for example a microphone turns vibrations (from sound) into an electrical signal. We can then send this analogue signal to a speaker and reproduce the sound. But analogue signals are susceptible to noise. For data transmission / communications it is better to use digital signals. Digital signals can only be one of a set of values while binary digital signals can only be on or off (high or low).
When noise is added to analogue signals it actually affects the data being transmitted, and produces distortion of the data. Digital signals still have noise added to them, but we are able to extract the signal being sent.
Binary digital signals can be processed by computers and transmitted so quickly that it is possible to send the information of a telephone signal quicker than the conservation is happening. This allows us to send more than one conversation down the same electrical cable at the same time. A similar process is performed with stereo digital music. This process is performed by a device called a multiplexer.
When sending a signal over long distances, its strength decreases with distance. If the signal is a changing voltage in a wire then this will be due to the resistance of the wire. If it is a radio wave it maybe due to it spreading out or it being partially absorbed. The noise added to the signal will become more significant and information more distorted. Repeater stations can be used to amplify the signal. Analogue signals will have any noise added to them amplified while we can 'clean up' the digital signal with no loss of data.
