Wikipedia:

Fiddleheads or fiddlehead greens are the furled fronds of a young fern, harvested for use as a vegetable.

Left on the plant, each fiddlehead would unroll into a new frond (circinate vernation). As fiddleheads are harvested early in the season before the frond has opened and reached its full height, they are cut fairly close to the ground.

Fiddleheads have antioxidant activity, are a source of omega-3 and omega-6 fatty acids, and are high in iron and fiber. Certain varieties of fiddleheads have been shown to be carcinogenic. (See bracken toxicity)

The fiddlehead resembles the curled ornamentation (called a scroll) on the end of a stringed instrument, such as a fiddle. It is also called a crozier, after the curved staff used by bishops, which has its origins in the shepherd's crook.