Modifers are a set of options that can be applied to your products.
Ex : select side dishes or add fillings or toppings in restaurants,
Ex 2 : attribute a vendor to a sale line,
Ex 3 : assign a specific status to a product line.

In 'Settings/Users/Access management' enable the right: 'Manage product modifiers'.

In 'Products/Product settings/Modifiers' you can create your modifiers grouped into categories (modifier groups).
A group of modifiers gathers similar modifiers.
First, create your group of modifiers (ex: side dishes) and then click on the magnifying glass icon to add the different modifiers (ex: french fries, potatoes, salad...).
You can assign a price to your modifier, that will alter the selling price of the product.
In each product form, in the 'Product modifiers' section, you can select the modifier groups that can be assigned to your product.

In a sale, select the product in the ticket, and choose the modifiers to add to the product.
In 'Settings/Misc' enable the 'Select product modifiers' button, to open a popup window when a product is added to a sale that will prompt you to select product modifiers.

Modifiers will print on the kitchen ticket, and can be printed on the receipt (see advanced settings of the receipt).

In the Reports tab, you will have statistics on modifiers consumption ('Reports/Date to date/Modifiers').