You clicked the tree somewhere and it would tell you either to try again, or you would win something. I think most people who won got $5 and a monkey plush toy. I'm not sure anyone ever won the jackpot. You could just click over and over again trying to remember where you had previously clicked, like a treasure hunt. Meanwhile they're showing banner ads on the page.

It worked using the ismap attribute on the image which tells the browser to add the x,y coordinates of the user's click to the link when fetching the result.