Search Engine Optimization is doing whatever it takes on your website, with help from external links and social sharing, to get your website listed in Google. Granted there are plenty of other search engines out there. However, Google represents close to 80% of all organic and paid traffic. If we focus on what Google wants, we'll get listed in those other search engines.
To me, SEO is like a Rubik's Cube. There's a whole bunch of moving pieces. Unless you know what you're doing, any move, no matter how small, will likely make things worse. And like a Rubik's cube, everyone you talk to will say they figured it out "accidentally one time." While those of us who know how to really solve it (and we do), know that unless you actually know the right moves, there's literally no way to accidentally do it.