You need to go back and get a good basic C book and start at the beginning. This isn't really meant to be an insult. But you don't seem to be able to do any problem solving on your own, or even basic deduction.

i is a counter that you use in your loop. Do you know what a for loop is? Well in case you don't, it goes something like this:
int i;

for( i = 0; an_array[ i ] != '\0'; i++ )
{ something here...
At any rate, I'd really suggest you find a good book and take it slowly. Don't move on until you have a really good grasp of the current section, and make sure you do all of the exercises there. Perhaps even try modifying them a bit to get a better understanding of things.