What does mean by "There should be no strong or weak (via Interface) reference of View in ViewModel" in MVVM

There is a MVVM rule like "There should be no strong or weak (via Interface) reference of View in ViewModel".

Currently I am converting one wpf application to MVVM and for that i am refering to rules as specified inhttp://michaelsync.net/2010/02/03/rules-of-mvvm.

So, i want to know what is strong and weak reference and how we can implement it. So once i get that knowlege i will be able to check this MVVM rule and hence i can make sure that there is not strong or weak (via Interface) reference of View in ViewModel.

So can anyone explain me?