Technical SEO: The on site stuff with relevant keywords in copy, heading/HTML structure, metadata, alt tags, etc. This is all stuff a good site should be implementing by default, but sadly a lot of them aren't.
External SEO: Link building. Getting attention from authorative sources so people know about your work and link to it.
In theory, a site that does these two things well should be better than a site that doesn't.
The problem is that while they correlate to a site having useful content and being useful to humans, they don't require it, especially when Google's systems are too stupid to tell if a site's content is any good and the company is deathly allergic to the idea of hiring people to check or applying any real penalties for wrongdoing.
So SEO is associated with all the crap sites that do these things badly in an attempt to do well in search, and Google's failure to filter them out (perhaps because it isn't actually possible to do so automatically).