Say you wanted to use the Quenya names for the sons of Feanor but were a bit concerned that people might not recognize them. So the first time you type the name Makalaure you enter something like
[a href="" title="Maglor"]Makalaure[/a]
... except that you would type a < where I've typed [ above, and a > where I've typed ]. (I don't want LJ to convert that text to HTML.) If the site lets you enter HTML directly - like the LJ new post page where you click the "HTML" tab rather than the "Visual Editor" one, or anywhere that you enter HTML tags to control the formatting rather than clicking a button. If you do change the formatting by clicking a button you can still use this trick, but you'll need to click over to the HTML editor option or in some pages, click the special button in the header that lets you enter HTML code directly. Anyway, once you get to where you're actually entering HTML, if you typed that bit of "Makalaure" text above you'd see Makalaure.
Which the website thinks is a link. It's not actually a link (if you click on it nothing happens) but if you just hover your mouse over it it would display the preview description you told the website to display. So if you need to include a short description the reader can see it without having to scroll to the bottom of the page and back again. Works great if you need to translate a foreign phrase or just include a short note without breaking the flow of the text.
Technical note: I know a few years back, some browsers wouldn't "wrap" the text meaning you'd only see the first line of the note. Don't know if that's still an issue, but the person does have to hover their mouse over the "link" while they read it so you don't want to make it too long.
Anyway, thought some of my fanfic friends would find that trick useful.