We’ll start with the mechanisms of speech production. Producing speech involves generating a source of sound, combined with a means of modifying (filtering) that sound. Understanding human speech production will directly help us understand what see see in acoustic speech signals, when we inspect waveforms and spectrograms.
However, although the acoustic signal is generally continuously varying, we perceive it as a sequence of discrete categories. So, we need a system of notation for describing those categories: this is phonetics and phonology.