A list box is list of values that can be selected by the user.  You can define either a Single Select or Multiple Select list box.  The single select list box allows only one selected item.  The multiple select list box allows many selections by holding down the Ctrl key.

Red Blue Green Purple Orange Yellow Black
In the Field Builder you need to provide the drop down values on the List Box Items tab.  You have two choices, Range and Item List.  A range is good for a age or year range.  The item list allows you to define any item in the list box as demonstrated above..