Although I work with online software a lot, Search Engine Optimisation isn't really something I get involved in. In fact, I've been on record as describing SEOs as "snake oil salesmen and charlatans". A bit harsh, but I've met too many that just weren't trustworthy.
SEO is also a very fast moving industry. I've not studied it for a few years so lots of the details I know are probably out of date.
But what I can say is that there are two major factors for your SEO. On-Page and Off-Page.
Basically, Google, Bing, DuckDuckGo, Yahoo, AltaVista - whichever search engine you use (DuckDuckGo is my favourite, Google has the best results) is a computer programme, called a crawler. It looks at a web-page, analyses it and stores those results in an index. So when you ask the search engine for something, it can look up those results in the index and show you stuff that matches your query.
On-Page SEO is all about making sure that your site is easy for the crawler to read. Making sure that the structure of your headings, paragraphs and other on-screen elements give hints to the crawler as to what the page is about. Because if the crawler doesn't understand the page, it's going to put it in the wrong place in the index.
Off-Page SEO is all about ranking. So when you type in a search for "Adidas Gazelles" - the search engine is going to look into its index and find thousands, millions of pages, referencing Gazelles. Which is the one that you actually want to see?
Off-Page SEO is used to try and filter that noise. Just because a page mentions Gazelles doesn't mean that it's worth reading. In the olden days, Google used a system called "Page Rank" to figure out which pages were the ones to look at - if lots of other websites linked to your page, then it was probably pretty good. The issue is that when SEOs figured this out, they would just stick links to pages everywhere they could find on the web, so Google would see all these links and boost the Page Rank accordingly.
Now, things are more complicated. In fact, proper Off-Page SEO is a task about brand management. It's about getting your keywords, phrases and brands seen in the right places. About proving that you have authority in that subject matter. Making sure that the search engine doesn't think you're trying to cheat the system.
In other words, On-Page SEO is pretty simple and can be done at the time that your site is built. Off-Page SEO is really complicated and is an ongoing, slow, process.