When you make an anchor, it effectively lets you place a "bookmark" on one part of the page. Chances are when you clicked on this thread to read this reply, the URL shows a # and something after it--this is one of vBulletin's built-in anchors, so that when you click on the thread it'll bring you to the exact place on the page where the newest post is, etc.
In the example I gave above with "bottom", it's basically adding #bottom to the end of the URL and that URL will bring you to wherever you set up your anchor on the page (which is done in BB Code with our [a id] tags). The [aurl] tags are just a quick way to get to them (so long as you're on the same page) instead of typing [url=https://www.pokecommunity.com/showthread.php?p=5654842#bottom]. (Actually, if you open my spoiler tag and click the link that brings you to the bottom of the spoiler tag, you'll see what I mean in the URL bar.)
So [a id]word[/a id] goes hand in hand with [aurl=word]text of the link[aurl]. And "word" can be anything you want so long as it has no spaces. That's all there is to it.
I... think that covers your question. @_@