According to Cuban, the vast majority of Google's revenue comes from Google AdWords. Google describes AdWords as follows:
"With Google AdWords you create your own ads, choose keywords to help us match your ads to your audience and pay only when someone clicks on them."Owners and administrators of web sites can allocate space on their pages for Google to place the ads that advertisers create. They receive a portion of the revenue generated when someone clicks the advertising links. Google receives the rest of the "click revenue".
Cuban drew attention to the fact that competitors can game this system. I.e., a competitor to an advertiser can write an automated script that repeatedly "clicks" the link, thereby causing the advertiser to pay huge sums of money to Google for worthless clicks (clicks by a machine that will never purchase anything). If this problem becomes pervasive, companies will no longer want to advertise with Google, and Google's primary source of revenue will dry up.