lolz, i guess you are kidding,
still, in case you are not, here I go :
SEO is the abbreviation of Search Engine Optimization. Search Engine Optimization is a broad definition for the various methods in which websites can be created, designed, and maintained with the intention of being indexed and ranked within search engines and online directories.
Search engine optimization (SEO) is the process of analyzing your site and modifying it to enable search engines to read it, understand it and index (or catalog) it correctly. This is not rewriting the site or changing the look and feel. It is subtle changes, adding or modifying inconspicuous visible and invisible text so that the search engines can read the site. SEO is not spamming the search engines - it is simply helping the search engines help you.
The need for SEO has become acute in the past few years. Web sites are being created with Java, Flash and images all of which search engines can not understand and ignore. If the content of your website is ignored by the search engines then they can't index your site,
If your site isn't indexed then no one will find your site by searching for it in Google, Yahoo, MSN, etc. If no one can find your site then your hard work and money spent creating the site is in vain.