As the name suggests, the pollen of the fennel plant. It is a common spice in Tuscany (where it's called the "spice of the angels"), though not much used outside of the region. The intoxicating fragrance is not just fennel, but also has flowery and spicy notes.
In Tuscany, fennel pollen is often paired with pork, either before or after cooking. Its anise flavor also goes wonderfully with poultry and fish. Try adding fennel pollen to your favorite spice mix or sprinkle a bit onto a finished dish—just a pinch gives a big boost of flavor.
Crushed fennel seeds can stand in in a pinch, but it doesn't exactly mimic the fragrance of fennel pollen.
Look for fennel pollen in specialty spice shops or gourmet stores.