As the web page becomes narrower, panels within an island will flow vertically, so that they remain visible rather than require a horizontal scroll action.
Here you can see a screen with multiple Islands (pink) containing multiple IslandPanel elements (blue).
See the Properties Help for more information on how you can control the layout of each Island Element.
To Create a new Island, Right-Click on the "Islands and Panels" node that is a child of all Screen Elements and select "New UI Island"
In each case, this activates a mouse pointer that you click down with the mouse button to describe one corner, then drag to complete the definition, releasing the left mouse button when the rectangle is how you want it: