The IslandPanel is for managing responsive web "blocks" of labels and fields. An IslandPanel, when generated into an HTML page becomes a DIV tag with specific CSS classes and styles that work together with those of the Island (which becomes a DIV also).
An Island Panel is a "horizontal" element while the Island is a "vertical" element. In broad terms, the Island is a row while the IslandPanel is a column.
Note that while the Island and the IslandPanel objects are roughly drawn to physical rows and columns on the source Screen definition, you can move an IslandPanel to a different Island using the Island's PanelOmitIncludeFilter property.