Binding keyword refers to what?
It's not a keyword, it's a markup extension.
Bindings always bind to the Path relative to the source, if there is no path they bind directly to the source, possible sources are the DataContext which is used implicitly if no other source is specified and ElementName, Source and RelativeSource. Please read the overview if you have not done so yet.
This is a binding to the Property DataContext of the window. If there is a list of elements in that, you will see it in the ListBox.
For example type DataContext = new List<string> { "A", "B", "C" }; in the constructor of your window after InitializeComponents(); and you will see that list in your ListBox.