主题：[求助]请教帮助中的代码问题？

代码如下：

Dim t1 As Date = Date.Now
Dim Sum As Long
For i As Integer = 1 To 10000000
    Sum = Sum + i
Next
Output.Show("计算结束, 耗时: " & (Date.Now - t1).TotalSeconds & "秒")

请教定义 i 和 sum 变量的意义是什么？？