Familiar language is that which the readers easily recognize and understand because they use it on a regular basis. One of the most important functions of language is to build "homophily" or a sense of commonality with one's readers. Language which is foreign and unfamiliar to the reader tends to emphasize the differences between writer and reader, and makes the message difficult to understand. By using language that is familiar to the reader, the message is likely to have more impact.
Consider the following examples.
An assignment given to a class of business students by their philosophy professor:
"The presently assigned paper necessitates an eloquently articulated analysis of the Existentialist perspective as it pertains to contemporary living. You should adumbrate the points which represent the sine qua non of your analysis."
A letter sent to high school students warning them of the risks of an unhealthy diet:
"Individuals who maintain a diet of high fat content are exposed to an increased risk of developing atherosclerosis, which is a buildup of fat deposits on the inner walls of the arteries. This condition can reduce or cut off the flow of blood in the arteries serving the major organs of the body. This can lead to poor health."