What is Add Multiple Quantity for modifiers?

The Add Multiple Quantity checkbox, if selected, allows the customer to choose multiple quantities of a particular modifier.

For example, a Build Your Own Pizza menu item with the Modifier Group 'Protein' can have multiple items under it (chicken, beef, anchovies, etc). By selecting the Add Multiple Quantity checkbox, customers can select multiple types of protein (chicken and beef) or multiple quantities of the same protein (chicken x2) for their pizza.