Basically Synchronous means if you have 3 ads on the page they load 1 by 1 i:e ad2 won't load until ad1 has finished..and so on, sometimes slowing the webpage down. Asynchronous means the Javascript is loaded into the browsers cache first so doesn't have to load for each and every ad so the rest of the ads don't have to wait for the ad before it to load, thereby speeding up the page load time. https://support.google.com/adsense/answer/3221666?hl=en-GB