Citing a source is a direct reference to an outside document, speech, text, device, etcetera, to add a sense of credibility and authenticity. Citing a source is most commonly seen as a direct quote from outside sources. Most of the time, citations are used in informative documents, persuasive/argumentative documents, news articles, analysis forms, etcetera.
For example, a news article may cite a quote from a scientist when they are reporting about something related to medicine, biology, etcetera. For example, "Doctor Man says 'some mushrooms are healthy while others aren't,' in the first paragraph of his article Mushrooms, Rockstars or Killers."