Top Articles are just what you think, the pieces being read the most that day.  However, there is a cap on how far back the system goes. We have articles that have had millions of readers so they would have taken up a lot of top spots long after they were first published - that isn't really fair to newer pieces that don't benefit from a good search engine spot.

So the Top Articles are actually recent top articles. They will change over the course of a day, sometimes drastically. If an article or forum post is being hit by spam, for example, the counter will go up even though no one actually read the article - when the article counter gets out of whack with the javascript counter numbers the spam counts are removed. So a piece being hit by spammers, even if they are not getting through, could go from first place to 20th when that happens.