Search Engine Optimization means the activities a webmaster need to do to improve a website's search position in organic (non-paid) search engine results which includes google, bing, yahoo etc..
1. Prepare the website technically
Make sure your website has proper indexing rules defined at server level so that crawlers/bots employed by search engines can find your website for them to index you site in their dictionary.
2. Preparing the content of your website
Make sure the content of your webpage is relavent to the targeted audiance. Try to find the trends/keywords used by website visitors when they wish to find similar relavent topics and try to include those keywords at meaniful places with in your content. Please note that, overdoing this can negatively affect the page ranking as serch engine algorithms are more advanced now a days to find keyword stuffing and similar tricks used by SEO content writers
3. Offline activities to market your website
There are several marketing efforts by which the website that will have a positive impact on SEO rankings. One of them include requesting other websites to link back to you websites when they mention topics that are also relavent with your website