It's basically a website that lists tropes, which are literary devices used in all forms of media. They have funny names for 'em, but you'll probably recognize them upon hearing their definitions. For example, "Adam Smith Hates Your Guts" is a trope for video games in which the economy is proportional to game progress, not to supply and demand. The economy hates the player, basically.
Another example is "The Scrappy", which is a character that is disliked by a lot of the fans, named after Scrappy-Doo. So tropes can apply to plot, weapons, characters, concepts, etc. Along with trope pages, there are pages for games, anime, TV shows, etc. that has a summary and a list of the tropes for that series. For example, the "Ace Attorney" page starts out with a summary and then all the tropes, starting with "Abhorrent Admirer".
There are also other versions of the pages that have different aspects. "Wild Mass Guessing" is a page for a series (or rarely trope) where people post theories about the page, "Just Bugs Me" is a page for a series where people post things that just bug them, etc.
"LaconicWiki" is a section of the site that makes the pages short and easy to understand. For example, "Pokemon" in LaconicWiki is "Gotta catch em all to be a master". Or the Wii's LaconicWiki page is "Nintendo's Third Option".
"SugarWiki" is a section that portrays things positively. A lot of times, it is used for games that appear happy but are actually very nightmarish. I won't give spoilers. "DarthWiki" is pretty much the opposite.
Go explore!