What does the variable "i" stands for?

Answer 1

From what I know C++ doesn't have a predefined variable called i.
I think you mean the variable i that is used in for loops. It is used because it's shorter for iterator and you don't have to worry if you declared it for something else (except when you have nested loops in which case j is used).
But i is mostly used because we got used to it. There's no rule telling you to use it. You can use any name you want

Answer 2

iterator

But any other variable name will also do, as long if it is no reserved keyword

Answer 3

From "Integer". Or "Index". Is is used as an index. Also in mathematics, it is used in sigma notation.