初学者求知道~Public Sub Withdrawal (ByVal Amount As Double )

菜鸟求知道~~Public Sub Withdrawal (ByVal Amount As Double )
Private mdblBalance As Double 

Public Proterty Get Balance() As Double 
  Balance = mdblBalance 
End Proterty 

Public Sub Withdrawal (ByVal Amount As Double )--- 这句话是什么意思，有什么用？？？

  If Amount >Balance Then 
  Err.Raise Number :=vbObjuctError + 2081 ,_
Descreption:="Overdraft " 这两条语句怎么解释。。。
  End If 
  mdblBalance = mdblBalance - Amount 
End Sub 

求各位师兄大大帮忙解释下，本人初学菜鸟。。求虎摸。。

[解决办法]
err.raise vbobjecterror+2081, "Overdraft"