The List Widget is for displaying and editing lists of values. it is primarily for the text array Fields that contain a list of values. It has this icon in the form builder Design mode toolbar:
When editing values in Browse mode a list editor is used to assist in specifying the values. Double click on the widget (or use the F2 key) to start editing.
List fields are not currently queryable using form based query, and only have the most basic operators in the query builder.