Inserting rows based on cell

Can anyone explain to me how this code works? I found it in an article.

Sub CopyData()
    Dim xRow As Long
    Dim VInSertNum As Variant
    xRow = 1
    Application.ScreenUpdating = False
    Do While (Cells(xRow, "A") <> "")
        VInSertNum = Cells(xRow, "D")
        If ((VInSertNum > 1) And IsNumeric(VInSertNum)) Then
           Range(Cells(xRow, "A"), Cells(xRow, "D")).Copy
           Range(Cells(xRow + 1, "A"), Cells(xRow + VInSertNum - 1, "D")).Select
           Selection.Insert Shift:=xlDown
           xRow = xRow + VInSertNum - 1
        End If
        xRow = xRow + 1
    Loop
    Application.ScreenUpdating = False
End Sub

Does it copy and paste rows from columns A to C based on value in cell D or does it duplicate it another way? I want something like this except I want to insert rows and only copy columns A and B. But if I don't know how to change this code so that column C also shifts down.