A "widget" defines how a shadow form will look like, and how it will deal with the components it contains.
Selecting the widget to use for the Shadow form
When using creating Shadow links, by default, you can choose between different widget types, that will determine how the form's components are displayed.
The above example, will create a "Node add" link that - once clicked - will show its three component all grouped in a single box.
Those are the widget types allowed at the moment:
single - used for a single component.
grouped - used for including many components, grouped in the same single container (no tabs)
tabs - used for including many components divided in vertical tabs.
If no widget is specified, tabs will be used by default.