Users are most likely to come across the term “digital signal processing” (DSP) while looking for a new pair of earbuds, home theatre systems or other audio electronics products. Some might think of it as a casual term used for marketing, but DSP is a very complicated subject. Digital signal processing involves taking an audio signal (it can be any signal but in this case it's audio) and digitally manipulating it to achieve the desired result.
Source: wiktionary