What is the actual task of CanExecuteChanged and CommandManager.RequerySuggested?

I got the following code from Josh Smith's MVVM tutorial.

Can anyone provide a quick explanation of what this code actually does?

public event EventHandler CanExecuteChanged
{
    add { CommandManager.RequerySuggested += value; }
    remove { CommandManager.RequerySuggested -= value; }     
}

I can't understand two things:

1. what does the CanExecuteChanged event do?
2. what does the CommandManager.RequerySuggested do?

The above code is from the RelayCommand Class from here.