The inventory feature is for when you wish to count down limited inventory stock. When inventory runs out, the item will no longer be available to buy on your menu. You can also press the pause button on an item to stop it from being ordered.
Inventory is set product by product.
Go to your Dashboard
Head to your Menus tab on the left hand side
Select a menu
Select a product via the Edit button on the right hand side of the menu categories
Click on the Track inventory slider
Enter how many units you are able to sell.
Click on the Save product to go back to the menu page
Turning off the inventory tracker will not stop the product from being sold.
For now, modifiers cannot be tracked. For example, even if you have run out of chips on your main menu, if given as a modifier they will still be available.