fill combobox with unique values.

Dear Experts,

I have below code to fill combobox with unique values. This code works only if data is in same sheet of combobox. My data is in sheet1 and combobox is in sheet2. Can some one please modify this code as per my situation.

In addition please write some descriptions also, to understand this code for me.

Thanks in advance

Private Sub Worksheet_Activate()
Dim r As Range
With CreateObject("Scripting.Dictionary")
    .CompareMode = vbTextcompare
    For Each r In Range("a1",Range("a" & Rows.Count).End(xlUp))
        If Not IsEmpty(r) And Not .exists(r.Value) Then .add r.Value, Nothing
    Next
    Me.ComboBox1.List = .keys
End With
End Sub