Dill seeds taste like a mix of fresh dill and cumin, warm and slightly aniseedy. They are widely used in Northern European cooking, for example in pickles, and in Indian cuisine.
I often use them in soffritti, where they lighten garlic and onion, with courgettes, mushrooms, beetroot and cabbage, and to perfume rice and grains. Toast them in a dry pan for a few seconds before using to release their full aroma. They also make a good substitute for shiso in some soups.