"Defining i" can be said much more clearly by saying "tell BoA that you are going to use a variable called 'i'".
So, firstly, I'm assuming you know what a variable is. It's a number (usually) that either changes in value, or that you don't know the value of. Think alegbra in the example I'm about to show you, i = 3.
Code:
i + 2 = 5.
i is the variable, since we didn't know what it was until we did the equation.
Now, in order to tell BoA that you are going to use a variable, you have to write it down in the script. So, under the variables; line, you'd write "int i;" (but without the quotation marks).
E.g:
Code:
begintownscript;variables; int i;body;
See? The "int" bit just tells BoA that the variable is an integer, or, basically, a whole number. You can also use the term "short", (by writing "short a;" for example, though I'd just stick to int for now.
Hope that's clearer...