Think about how you use the web and how you search for things. Do you look past the first page of search results? Do you read the description under the URL to find out whether the page is what you are looking for?
Search Engine Optimisation (SEO) is about building a web page that will rank high in the search results and contain the right information in the search results that will make people click through to your page.
It is a combination of the way you build the page (eg headings) and the words you include.