What this Private Sub does?

[marlonsaveri]

Private Sub Worksheet_Change(ByVal Target As Range)
    Dim cellContents As String
    Dim valLength As Integer
    cellContents = Trim(Str(Val(Target.Value)))
    valLength = Len(cellContents)
    If valLength <> 3 Then
        MsgBox ("Please enter a 3 digit area code.")
        Cells(9, "C").Select
    Else
        Cells(9, "C").Value = cellContents
        Cells(9, "D").Select
    End If
End Sub