Traditionally SEO as the term indicates was the process of analyzing content for a site, using tools to select the best keyword phrases and to use those phrases throughout the content of a page and site.  It also meant building relationships with other websites, especially authority sites and somehow getting those sites to host a link to the page or site being optimized.  In simple terms, the combination of the optimized keyword phrases and nexus of the content, and the sites that link to it, helped search engines, firstly Google, make an algorithmic determination of the importance of the site in its nexus to the topic or keyword phrase.  The more important the page or site, the more it is considered a hub of knowledge that search engine users may be seeking.  All this is used by Google to set a value for a metric called pagerank.   This pagerank value gives us some indication of ranking of the page vs other pages on the internet for that keyword phrase.  Maximizing pagerank for keyword phrases is the objective.