Keywords are basically the words someone would use when typing into a search engines. So if I were looking for a web designer and I typed 'web designers' into a search engine my keywords (or keyphrase) would be 'web designers'
If I had a site and wanted it to appear in search results when someone typed in 'web designers' into a search engine I would use the words 'web' and 'designers' on my web page in different elements on the page.
With AdSense if you write a page that is mostly about 'web designers' your ads will probably be related to 'web designers' You might get ads that appear for different web design companies.
Adsense won't specifically show the keywords you use on the page. AdSense will show ads that are related to the keywords on your page and your website as a whole.
Basically if you want ads that are about electric guitars to be displayed you should write a page about electric guitars.
I'm not sure if that helps. I'll be honest that I'm not entirely sure what you're asking. If I got it wrong just provide a few more details and I'll be happy to try again.
