The top bar allows you toggle between different responsive breakpoints, undo,redo all design operations.
The Overall topbar

Page indicator — shows which page you are currently on. And when you click this indicator, it opens the Pages Panel.

Page Indicator

Breakpoints buttons — located in the left of the top bar, these icons allow you to switch between different breakpoints to see and edit the design of your project on these breakpoints (Desktop, Tablet, Phone). When you can design anything in Breakpoint mode - it will also automatically generate media queries.

Show Borders — This allows you to view the structure of your layout. For example, the grid overlays guide you to create a consistent design so your elements are stacked and positioned evenly throughout the site.

Undo/Redo — allow you to undo or redo actions you took in the Designer like applying a style or deleting an element.