A Spore is the reproductive structure of a variety of organisms including fungi, plants (such as ferns), bacteria and algae. Spores are usually adapted for dispersal and can be ejected from their organism and carried through the air over long distances. When conditions are favorable, the spore can begin to develop into a new organism.
Spores differ from seeds in that they do not have their own food reserves. This makes them less susceptible to animal predators but more dependent on favorable conditions in order to successfully germinate.
The bright orange circles on the underside of the Sword Fern are it's spores.