Why are fossil fuel resources sometimes referred to as "buried sunshine"?

In essence because the fossil fuel is comprised of animal and plant matter that died many millions of years ago. According to the food chain, the sun, our nearest star, is the source of most energy on the planet earth and thus "powering" (very simplified) plants that were eaten by herbivores, which in turn were eaten by carnivores. All these three groups can turn to fossil fuel - hence the term "buried sunshine".