Mustard seeds are tiny, flavorful seeds from mustard plants, used whole or ground for spices, condiments like mustard paste, oil, cattle feed, and in religious parables, symbolizing great growth from small beginnings. They are rich in protein, fiber, vitamins (B, E, K), minerals (selenium, magnesium, iron), and healthy fats, offering potential antimicrobial and antioxidant benefits, but should be consumed in moderation due to high fiber and potential goitrogenic effects.