请大家指点一下 visaul basic .net中的checkedlistbox控件的一些内容

  Dim i As Integer
        a = TextBox1.Text + "" + vbCrLf + a + vbCrLf + "专业:" + ComboBox1.Text + vbCrLf + "简介:" + vbCrLf
        For i = 0 To CheckedListBox1.Items.Count - 1
            If CheckedListBox1.GetItemCheckState(i) Then
                a = a + CheckedListBox1.Items(i) + vbCrLf
            End If
        Next
        MsgBox(a)

我不太清楚第3,4,5行代码的意思和作用..