When the internet was young and search engines were simple, it didn't take much to get your website to the top.
The search engines would look for a keyword, let's say: SEO. The more times your website said SEO, the more important the search engine assumed it was.
People would repeat the word hundreds of times - often in the same color as the background so that it wouldn't bother the actual human visitors.
Although this was one of the first SEO tactics that search engines evolved past, it persisted in the website design of individuals who had fallen behind the times. As a result, their websites fell behind the times in traffic - so to speak.
These days, anyone who claims to fully understand SEO is either working at Google or exaggerating a bit. There are good strategies, but even the professionals occasionally get together and ask each other speculative questions about what may or may not impact rankings.
I think its fairly simple: Get good content, link to the content, and then add some more content! Stay on topic and find your spot on the internet with links coming in and going out to related, trusted sites.
