It checks to see if the welcome table matches 'Welcome Guest' to ensure that the user is a guest. Though it would be easier checking pb_username but idk if that was there in 2005.
If the user is a guest it then loops through all the tables and checks that the inner html matches 'Info Center' and has a width of 100%. If matches both those things it sets the table display to none.
If you wanted the 5th on it would be document.getElementsByTagName("TD");
Arrays start counting at 0 so the 5th element in the array would actually be the 6th td element on the page.
The second reason is so you can use the modulus operator on the array which is extremely useful if you are using your array as a circular container meaning once you go past the last element in the array you loop back to the first element. The is useful in data structures such as hash tables and circular queues. Just google the modulus operator if you want to get an idea of what I'm talking about.
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