Most of the plants we notice are flowering plants, also known as angiosperms. Flowers are reproductive structures and, once pollinated, will produce fruits containing seeds that form the next generation. Ferns do not produce flowers or seeds, but instead, reproduce via spores.
When a fern is mature, it will develop sori on its leaves, often on the undersides, and these release spores into the air. Once a spore alights on a damp surface, it germinates and grows into a green fleshy structure that is often heart-shaped, called a gametophyte. Organs, called antheridia, on the surface, produce sperm, which swim across the ground searching for an organ called an archegonium, which bears the eggs. Once fertilized, a new fern develops, ultimately producing spores of its own. In order for sperm to travel successfully, they must have a film of moisture, and so most ferns can only reliably reproduce in damp habitats or during rainy periods.