A seasoning widely used in southern Italy, especially Sardinia and Sicily; it's made of the roe of either tuna or grey mullet, salted, pressed and sun-dried.
I know that it's sold either grated or whole (in the shape of a sausage) if your recipe calls for thin slices, but I've seen only the grated kind; the bottarga appears as a coarse, slightly wet sand, of amber/brown color.
You should keep it refrigerated to preserve its smell, but it won't spoil if left in the cupboard.
The grey mullet bottarga (bottarga di muggine) is rather expensive and has a delicate flavour; the opposite is true for the tuna kind (bottarga di tonno), whose smell is rather pungent: you must use it more sparingly, or its taste will overwhelm the rest of the ingredients.