Organic SEO (search engine optimisation) is a term that refers to getting your website to the top of the search results based on the content on your site, as opposed to paying to appear at the top of the results using a service like Google AdWords, for example.
How is organic SEO achieved?
If your website is “content rich” (i.e. full of plenty of useful pages/posts, etc.) and that content is full of the “keywords” you’re targeting, then Google sees your content as having a real benefit to the visitors to your website. This in turn results in your website ranking higher, resulting in more traffic. Of course, to keep visitors coming, you need to keep creating engaging content.
It will also be far more likely that other websites will link back to your engaging content, and this will also be a benefit to your search ranking (as Google will see your website has being of more importance if others are linking to it). However, it has to be done properly. Google is smart. It’s not OK to just create lots of uninformative content and then link back to it from lots of unrelated websites; that will actually do you more harm than good.