Present Value Distribution of Cash Flows

Reading 28, Vol 4, P16, PVDCF (Present Value Distribution of Cash Flows) It is stated that PVCDF is “a list that associates with each time period the fraction of the portfolio’ duration that is attributable to cash flows falling in that time period”. I don’t understand this definition ! The calculation described by the 4 steps is confusing too. And what are meant by those statements in the last paragraph of this page ? Can anyone help ? Thanks !