Here is one video of Erik Mork who explains how commanding works in Prism and MVVM.
Prism Commands allow designers/develops to specify, in XAML, events that fire back into the ViewModel/Presenter. Imagine this: the user hovers a mouse over a Silverlight control. Commanding allows that action to be bound to a method (more specifically, a delegate) in the ViewModel. The ViewModel is then able to handle that action.