Without getting lost in the technical specifics, the simplest way to understand SEO is to look at two things: what SEO does for your website, and what search engines, Google in particular, want to see from it.
One: what SEO does for your site
Good SEO paints a clearer picture of your site in terms of usability, accessibility, and content, and delivers an easily recognized solution for the visitor. A big part of that is regularly refreshing and adding content so your site stays relevant and reads as a genuine resource for the person searching.
Two: what search engines want
Search engines see their searchers as customers, and they want to serve those customers the best possible answer. So when you are shaping the SEO or content direction of your site, whether you are optimizing images, titles, page content, or target phrases, the single biggest factor is whether you are bringing real value to the visitors you are trying to reach.
Put simply: add targeted, valuable content, follow the compliant standards for how a page should be built, and keep both your reader and the search engine's perspective in mind. Do that consistently and you are in good shape.
Developer's advice
Pick one question your customers ask you constantly and write a genuinely useful page answering it. That single honest page will usually outrank a dozen keyword-stuffed ones, because it is exactly what Google is trying to reward.