Mustard seed was a seed taken from an Earth plant, that, when ground up, is 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 black pepper, paprika, mustard seed, and cayenne, to B'Rat Ud for the formula to synthesize trellium-D. (ENT: "Rajiin")