Mustard seeds are the small round seeds of various mustard plants. Mustard has a strong pungent flavor, and unlike any other spice. Mustard seeds are rich in a nutrient called selenium, known for its high anti-inflammatory effects.Benefits - Mustard seeds are rich in a nutrient called selenium, known for its high anti-inflammatory effects.The high source of magnesium in mustard seeds helps reducing the severity of asthma attacks and certain symptoms of rheumatoid arthritis and lowering blood pressure.