Signals are things like vibrations, sound, pictures, brain waves, seismic activity, radio waves, light transimissions etc.

Digitising these signals turns them into long strings of numbers.

Digital signals can also be generated directly using relatively simple mathematics.

Processing signals as strings of numbers is far easier and more flexible than processing the raw analogue signals.

Once a signal has been digitised, processing can be done with a computer program.

Computer programs have the advantage of being much easier to modify and allow more complex mathematical manipulations.