Wikipedia
Mediabot is the name given to the web crawler that Google uses to crawl webpages for purposes of analysing the content so Google AdSense can serve contextually relevant advertising to the page. Mediabot identifies itself with the user agent string "Mediapartners-Google/2.1".
Unlike other crawlers, Mediabot does not follow links to discover new crawlable URLs,instead only visiting URLs that have included the AdSense code. Where that content resides behind a login the crawler can be given a login so that it is able to crawl protected content.
Mediabot will usually first visit a page within seconds of AdSense code being called from that page for the first time. Thereafter it revisits pages on a regular, but unpredictable basis. Changes made to a page therefore do not immediately cause changes to the ads displayed on the page.
Ads can still be shown on a page even if the Mediabot has not yet visited it. In this instance ads chosen will be based on a combination of the overall domain theme and keywords appearing in the url string. If no ads can be matched to the page, either public service ads, blank space or a solid color are shown, depending on the settings for that ad unit.