Search engine optimization is the process of doing certain things so as to rank higher in the search engines - particularly Google, MSN and Yahoo.
If you get listed higher in these three search engines for the search terms that match what your website is actually offering, then you will receive free targeted traffic whose conversion on Adsense will be very good as the ads will match what they're looking for.
The aim of search engine optmization is to tell the search engines what you are relevant to - you are basically trying to tell them which search terms you should rank highly for.
The content on your webpages will already be doing this for you. For example, this page is relevant to "search engine optimization", whereas my home page is relevant to "making money with Adsense". Each page is about a different topic and will therefore be related to different search queries but there are various techniques you can use to stress each pages relevancy to different keyword phrases.
This is called Onpage Optimization and includes:
1. Including a keyword rich title on the page,
2. The appropriate use of Meta Tags,
3. Using Header Tags for your main and secondary keywords,
4. Including your keywords in the content of your page,
5. The appropriate keyword density and placement.
By doing all of this, Google will therefore know that my home page is about "Making Money With Adsense" but how does it determine how valuable it is in relation to other websites which stress they too are about "Making Money With Adsense". How does it determine which order these websites should appear in when someone searches for that term.
Here the second part of seo comes into play which has been named Offpage Optimization or Link Building.
By analyzing the linking structure of the internet the search engines hope to work out which websites and which webpages are more valuable. A link from another site to yours is considered as a kind of endorsement - a vote in your favour.
The aim with link building is to have a good supporting linking structure which pushes you up the search engine rankings and holds you on the first page. There are many ways to get inbound links to your website including:
1. Submitting your website to directories.
2. Exchanging links with other websites (this is called reciprocal linking).
3. Including your website in your forum signature or when you leave comments on other peoples websites or blogs.
4. Submitting articles (which include a link to your site) to many of the internets article directories which other webmasters then take and place on their own sites.
5. By buying text links from one of the internets many text link brokers.
6. By advertising on or becoming a sponsor of another website.
But beware - before you spend months trying to get links from every website you come across the twist in the tale is that not every link carries the same weight. What you want is high value links. The factors which affect the value of a link include but are not limited to these:
-                                                             Whether the website                                                             is a highly regarded                                                             website - an "authority"                                                             site.
                                                            - Whether the website                                                             and it's webpages                                                             have a high page rank                                                             (note that page rank                                                             can be a good indication                                                             of value but is not                                                             absolute in its accuracy).
                                                            - Whether the title                                                             of the page and it's                                                             content matches or                                                             is related to your                                                             target keywords
                                                            - How many other external                                                             links there are on                                                             the linking page -                                                             there should be no                                                             more than 20 links                                                             on this page
                                                            - The number of steps                                                             from the home page                                                             that the link page                                                             is. 
                                                            - The anchor text                                                             that you can use in                                                             the link - you want                                                             to be able to use                                                             your target keywords                                                             and not your business                                                             name.
                                                            - Who links to the                                                             page and to the site?                                                             
Please note that this page is an inevitable simplification of search engine optimization in order to give a basic grasping of it. Search engines are constantly evolving the ways in which they rank websites and so search engine optimization must too constantly evolve. It's importance in relation to Adsense is it's ability to drive LOTS of free and targetted traffic to your website and as this websites continues to grow we will cover it in much more depth.
