SEO is the idea that the search engine can be gamed into ranking your content. It can't. Google has the biggest cache of information on planet Earth, so they know if your content sucks or not. How, exactly? They measure engagement, pages read per visit, per visitor - - a precise and reliable metric of relevance.
Historically, Google was reluctant to rely heavily on visitor engagement as a ranking signal because it would incentivize click fraud for those seeking to game rankings or inflate ad revenue. However, as Google's knowledge base has expanded, they can tell exactly WHO is doing all of that clicking. As they control click fraud, their ability to rely on engagement as a ranking indicator has gone up.
Bottom line, you can't fool Google, which is why we trust Google search results. They know all about how your visitor interacts with your website. If you doubt this, check out your own website metrics in Google Analytics. Who knew they knew so much…
The takeaway? Just put your focus on taking care of your web visitor and Google will take care of you. Regarding SEO, certainly you should use common sense in tagging and weighting your content, but ultimately your ranking will depend on engaging the specific concerns of your targeted visitor. Since Google's business model depends upon your great content, their algorithm will always find a way to reward you for developing it. Publish engaging content on a regular basis and you will see your overall rankings go up, up and up.