Mustard gives a dressing sharpness and helps the ingredients come together. It can also add depth to a pan of beans, but the strength differs considerably between varieties.
Try half a teaspoon in a small lemon-and-oil dressing, whisk, and taste before adding more. A smooth strong mustard behaves differently from a mild wholegrain one. If a recipe already includes vinegar, account for the mustard's acidity as well.
Check the ingredients when buying: some varieties include honey, wine, or other additions relevant to your preferences. Follow the jar's storage instructions after opening. Mustard is also an allergen for some people, so do not hide it behind the vague phrase “a little seasoning” when describing food to a guest who needs a precise ingredient list.