A quick google search would give you the answer but here is my attempt to verbalise it.
The list box is used to point at table fields and will then display all items within the field once.
This can be useful in a number of ways as you can use it to filter data easily and still see what is filtered.
Using 2 or more lists boxes for analysis can be useful when getting to know your data as the association engine will show you what items exist in one list box based on the selections in another.
i.e. 2 list boxes for Continent and Country -> Select Europe (It becomes Green in the first box where Green is a Selected item) in Continent and European cities will be white (items that are in the current selection) in the Country box and non-European boxes will become grey (not selected based on the current filter). You should see this in demo QWV files provided by Qlikview or load some data you are familiar with into Qlikview and play around with it.
There are many other ways to use them but the above can be one of the most useful for beginners and highlights the power of Qlikview as a data tool.