Understand WPF Binding CommandParameter="{Binding}"

An Empty {Binding} will pass the current DataContext of the control to the Executed and CanExecute methods respectively.

{Binding ...} is a MarkupExtension.
In its usual form it takes a Path like {Binding Path=someProperty, ...} (or its short form {Binding someProperty, ...}).
So the path in {Binding} is empty which means the Binding is bound to whatever Source there is for the Binding. This might be a little easier to understand if you know, that {Binding} is actually the same as {Binding DataContext,RelativeSource={RelativeSource Self}}.

So in your case CommandParameter gets the value of the current DataContext of the Button.