Internal Optimization means on-page optimization that you should take into consideration while writing your internal web pages. This is one thing you have complete control over. Besides, it has a major effect on the ads relevancy and CTR.
Meta Keywords: They have low impact, BUT – they DO have impact over the ads relevancy. You are advised not to leave this part of your page as blank. Always try to make it as easy as possible for the crawlers to understand your website. Create your web page with Meta keywords. If there is NO CONTENT on your webpage, the Adsense Media bot will pick-up the Meta Tags, and show ads according to that. Search on Google for pages that top the SERPs for the keyword you have in mind. These websites will give you a good idea about what to include in Meta Tags.
Meta Description: Whatever you put in the description meta tag, it will appear as description right after the Title Text that links to your page in the SERPs (Search Engine Result Pages). This property of the description meta tag makes it quite important, as it has direct impact over the CTR of your link in the SERPs that leads to your website. It eventually decides how much traffic you will get from a search engine. Write description that attracts right kind of traffic from search engine pages.
Search Engines give importance to the keywords that appear in the title tag of HTML Source code. Title has a good impact over relevancy of ads too. For higher payouts, choose the highest paying keyword of that category for the title text.
However, this entirely depends upon your optimization strategy. If you are more concerned about competition and getting more traffic than affiliate marketing payouts, choose a keyword with low competition keyword (competition you can handle) with relatively higher traffic. Though, I will advise you to get more traffic first, and become authority page of that keyword. Payouts will increase with the traffic.
Tip: Title text generally appears in the SERPs as TITLE link to your page. In other words, you can say, this very line controls the CTR of your affiliate marketing website link listed in SERPs. How you design your title is entirely your own decision.
If you are optimizing it for more traffic, you will not want your title to read as it was written for Search Engine crawlers. The title has to be concise so much so that even 10 words are considered as too many. If you are optimizing it for more payouts – Go ahead!
However, you can try to mix the high paying keywords with traffic attractor keywords too.