Mustard seed was a seed taken from an Earth plant, that, when ground up, was used as spice used to make, among other things, the Human condiment of the same name.
In 2153, Jonathan Archer exchanged several spices from the ship's galley, including cayenne pepper, mustard seed, paprika, and black pepper, to B'Rat Ud for the formula to synthesize trellium. (ENT: "Rajiin")