A Googlebot or "spider" is a script sent by Google to your website to collect data. This information is then sent back to Google and is used to determine what type of ads would be relevant to your website. A Googlebot reads text only and not images.

Sometimes Google cannot find advertisers for your AdSense ads. This may happen if the Googlebot cannot not figure out what your page's topic is about. This may occur:

if your article is too short in length. The more content the better.
if your article has a low keyword density. Insert relevant keywords into your article whenever possible.
if your article is about two or more topics. Try to stick to one topic per page.
if the Googlebot is picking up page information irrelevant to your article such as your website's profile, previous posts, sidebar, comments etc. Use section targeting to solve this problem.

Use section targeting. Google allows you to enter two html codes. One let's you specify what content you want AdSense to focus on. The other one let's you specify what content you want AdSense to ignore. For more info click here: https://www.google.com/adsense/support/bin/answer.py?answer=23168 . What I do is rearrange my article. I place the paragraphs with the highest targeted keywords at the top of the article and use section targeting to place emphasis on these sentences. I use section targeting to ignore the rest.

Visitors will not click on ads that are not relevant or do not interest them.

Don't chase "high paying keywords". Concentrate more on "high traffic keywords".

Sometimes Google cannot find advertisers at all for your AdSense ads. They will insert PSAs (Public Service Announcements) instead of AdSense ads. This also causes you to lose clicks.

The AdSense Troubleshooting wizard will help you if your AdSense ads do not match your blog's topic or you are receiving PSAs.