Default selections are the pre-selected modifier items that show up in the menu.
You might want to set up default selections to some of your modifier items so that while the customer is placing an order, they can see them already selected or can unselect them to choose different items instead.
Initial Steps
1. Be on Control Panel,
2. Go to Menu tab
TABLE OF CONTENTS
How to set up Default selections?
So let's go to the toppings modifier that we created in the article - 15. Modifier Grouping and let's say the toppings modifier comes with American cheese, Onion, and Tomato selected by default so when a customer goes to the item, they can see them selected.
Setup
1. With the Toppings modifier selected, go to Toppings Modifier Items... and
2. Click on Default selections
3. Check-in the items
4. Save it
The default selections are shown in the blue checkmarks.
How it appears in the Order online menu?
Go to the Order online menu > WEDNESDAY SPECIAL > Specials > BBQ Chicken Pizza
Now toppings come collapsed that's because you only have a max of 4 toppings. And you already have 4 selected, so it shows up collapsed to the customer because there's no need to make any more selections.
But they can go there and unselect them to choose a different topping by clicking on the collapse/expand button 
as squared in the previous screenshot.
If you want the modifiers to show expanded, these can be controlled through the configuration settings. So talk to your account manager if you want the modifiers to always show up expanded.