Modifiers are similar to menu items but they exist elsewhere in the menu. They modify menu items by content or size.
Let's say you have a build-your-own salad menu item where customers can choose a base, toppings, and dressing. The base, toppings, and dressing are all the Modifier Groups and the specific selections within each group (e.g. kale or arugula for base) are the Modifiers.