"Google Adsense Spiders" are just one of the many Spiders that you'll find all over the World Wide Web these days. It's not a name I like because it automatically triggers thoughts of bugs, or even worse, viruses...but that's not the case and they actually do a very important job.
In short, spiders crawl the net creating indexes of all the pages and their content to allow you to be able to use search engines to locate the information your after.
The "Google Adsense Spiders" have a slightly more specialised job in that they are tasked with looking at the content of a page and feeding that information back to "adsense" who then serve up (where possible) relevant adverts based on the page content.