Google AdSense works by displaying ads relevant to a page. To get relevant ads, Google AdSense crawler crawls your site and extract keywords from the pages it visit. When a visitor visits a page on your site, Google will serve ads based on the extracted keywords that Google AdWords advertisers bid on.
With that in mind, it’s important for Google AdSense crawler to get the right keyword from your page, as if it doesn’t, it will either display irrelevant ads or worst, PSAs or Public Service Avertisements. Irrelevant ads gets less click from your site’s visitors, and PSAs don’t pay at all.
Keyword targeting for Google AdSense in a sense is similar to keyword targeting in SEO or Search Engine optimization, with some unique targeting features of its own.
