SEO is one of those terms you see a lot on the web and, sooner or later, someone you know will mention it, especially in the work context. It’s an acronym for Search Engine Optimisation, otherwise known as "positioning" or natural referencing, and it means all the free techniques a blog or website can use to be among the first results that come up on a search engine (Google, Bing, Yahoo, etc). SEO works like a sort of shop window. You have to display your products in the window to attract the passing trade and induce the customers to come in, if you want to drive up numbers of visitors. It’s the same for a website: you can present the site using keywords that will attract the attention of internet surfers and search engines. For example, the words featuring in the URL address of the website, or in the description (signalled by a meta tag). These are known as "internal" techniques, but there are other, external ones in SEO: for instance, you can post comments on forums or popular blogs, and add a link to your website in the hope that people visiting that forum will be attracted to your site. You can also publish messages on Facebook, twitter, Linkedin or other social networks, to get your website better known.
SEO is a practice permitted by the search engines. Just as a shop owner has the right to bring in customers by placing attractions at the entrance to the shop and finding a talented window dresser, the owner of a website has the right to communicate the existence of that site on the web, and that means presenting it using words that enhance its visibility.