Breakpoint Variants is a Creator Studio Responsive feature which will allow you to create groups of Experiences targeting different screen sizes by using Breakpoint Variants.
Your web site Breakpoints are the points at which your site content will change to provide the user with the best possible experience to match the viewport width of the screen where the Experience is displayed. This depends on the screen size from where the user is accessing the content (smart phones, tablets, desktops).
Set Up Breakpoints for Your Account
To configure this, open the Settings menu in the top right corner and select Set Up Breakpoints. This is a universal, one-time configuration defining the screens you want to target - shown in the image below:
The default setting is 2 breakpoints, 768 pixels and 992 pixels. These are minimum widths defining the intervals to which you can assign different Breakpoint Variants in the Editor. The maximum number of breakpoints you can define is 5.
From this section you can add, edit and remove Breakpoints.
For more information on how to create and assign Breakpoint Variants, check out this article here