William Faulkner used this phrase as part of his Nobel Prize of Literature speech banquet in 1949.
What he means by this quote is that the best expressions in written language are often the most simple, sincere, and natural that can be found. Love, pride, compassion, and sacrifice are examples of the most basic emotions of human nature.
Why adulterate or water down literature using topics that are much too complex to understand or too tedious to even discuss? Why look for more problems within reality when our every day life gives us so much to marvel at and cherish deeply?
Old universal truths are constants, not variables. They are so strongly anchored within our psyche that they will hardly ever change or change our perception of them. The basic elements of humanity are to be explored and celebrated. There is no artificial additives in good literature. Anything that is true, pure, and simple is great. Anything that attempts against that is simply violating any artistic canon of literary beauty.