求vb倒时计时器代码解释

帮我把这个代码解释写出来。在线等。谢谢。
1、在窗体上添加一个label1控件
2、在窗体上添加一个command1控件
3、在窗体上添加一个timer1控件
4、复制如下代码，运行
Dim a As Long
Private Sub Command1_Click()
a = InputBox("请输入秒数")
Label1.Caption = a
Timer1.Enabled = True
End Sub

Private Sub Form_Load()
Timer1.Interval = 1000
Timer1.Enabled = False
Command1.Caption = "记时开始"
End Sub

Private Sub Timer1_Timer()
a = a - 1
Label1.Caption = a
Print a
If a = 0 Then
Timer1.Enabled = False
End If
End Sub

Dim a As Long
Private Sub Command1_Click() //触发click事件
a = InputBox("请输入秒数") //输入倒计时秒数
Label1.Caption = a //显示a
Timer1.Enabled = True //启动计时器
End Sub

Private Sub Form_Load() //先执行
Timer1.Interval = 1000 //初始化，时间间隔1秒
Timer1.Enabled = False //计时器禁用
Command1.Caption = "记时开始" //让按钮显示“计时开始”
End Sub

Private Sub Timer1_Timer() //每隔一秒执行一次
a = a - 1 //计数器
Label1.Caption = a //显示剩余秒数
Print a //显示a
If a = 0 Then //控制循环，到a变为零即计时结束
Timer1.Enabled = False
End If
End Sub

昨天在网吧写的代码没有了,我郁闷啊. 今天把界面丰富了一下,你看看吧,不知道你说的启用键盘回车功能干什么,所以没有涉及到.仍然要感谢rainbow_route.窗体中一个textbox,三个command,一个timmer,看不到字就把textbox弄大点. private declare function gettickcount lib "kernel32" () as long public tstart, tnow, tlast, tminute, tsecond, ttotal private sub command1_click() command1.enabled = false: command2.enabled = true: command4.enabled = false text1.text = "" end sub private sub command2_click() command3.enabled = true timer1.enabled = true ttotal = tlast + ttotal tstart = gettickcount call timer1_timer end sub private sub command3_click() command1.enabled = true: command2.enabled = false command3.enabled = false: command4.enabled = true timer1.enabled = false tstart = 0: tend = 0: ltime = 0: tminute = 0: tsecond = 0: tlast = 0 end sub private sub command4_click() end end sub private sub form_load() command1.caption = "开始": command2.caption = "计时": command3.caption = "停止": command4.caption = "退出" command1.default = true: command2.enabled = false: command3.enabled = false timer1.interval = 1000: timer1.enabled = false tstart = 0: tend = 0: ltime = 0: tminute = 0: tsecond = 0: tlast = 0 end sub private sub timer1_timer() tnow = gettickcount tlast = val(tnow - tstart) \ 1000 tminute = tlast \ 60 tsecond = tlast mod 60 text1.text = "距你本次点击[计时]的时间为" & tminute & "：" & tsecond & vbcrlf & "从第一次点击[计时]到本次点击经过了：" & ttotal & "秒" end sub