The idea is that you're shown words or short phrases on the screen and you want to quickly type in the first word that comes to mind - a typical word association. It showed "chess" and I typed "checkers". It showed "never cease" and I typed "to amaze". You get the idea... but be forewarned: once you start blazing through some words, it makes you want to keep going to see what it displays next.
The coolest part of the site is actually on the next tab: View the Cloud.
Here, you see a set of balls, each with a word on it, and as you type in a word the balls begin to disappear - revealing only the balls that match what you've typed. Having narrowed down to one or more manageable balls in the display, you can click on one of them to expand it into a network diagram. The ball you click then "explodes" into a set of balls that match words people typed in during the the word association process. The thicker the line connecting the two, the more common the association between the two balls (i.e., between the words on the connected balls).
In the image to the right, I typed "sql" - which narrowed down to just one ball - and then clicked on it to expand the associated words. The thickest lines are to "database" and "query", followed by "my" and "server". Slick. You can follow the word association visually by clicking on any associated ball to reveal its associations... and so on. To make the display manageable, balls begin to shrink and fade out over time as you drill down into other associated words.
Aside form being a bit addictive, it's also an entertaining visualization. Pure infoporn.
Their blog has a great entry with some funny stats and insights from the word associations people have entered (at this point, about a half million words with over 6 millions connections).
They're working on a game called "World of Goo" that (from a preview video) also looks like it'll be pretty cool.