비주얼베이직입니다 코드 해석점 해주세요

Private Sub Command1_Click()
Dim a, b As Integer
Dim c As String
st = t1.Text
en = t2.Text
For a = st To en Step -1
    c = c & CStr(a) & "+"
    b = b + a
    
   Next
   c = Left(c, Len(c) - 1)   ======>이게 무슨말입니까
   c = c & "=" & CStr(b)
   out.Caption = c
End Sub