SEO is one of the most misunderstood and mysterious elements of web design. SEO stands for Search Engine Optimization. That's techspeak for making your website attractive to Google, essentially. Yes, there are other search engines... but Google is the big one everyone uses.
There are two main categories of SEO work: on-site and off-site.
Creating valuable content that incorporates high-value keyword phrases is the key to on-site SEO. This can mean either blogging on an on-site blog that's incorporated into your site (Wordpress and Squarespace both offer this) AND creating valuable content on your static pages. Focusing on a specific keyword phrase for each page helps that page climb higher in search results. There are other ways to add that phrase to the page: image file names, captions under images, alt tags on images, metatags, H1 tags in the HTML, and in the actual URL slug(the end of the URL for that page) are all places where that phrase can be duplicated, resulting in solid usage of that phrase on that page. Outbound links are also helpful.
Part of my web development process is researching keyword phrases for the client. From those phrases, I am able to isolate a few that we focus on. A main keyword phrase (usually 3 to 4 words) and a few supporting keyword phrases. Google even suggests related keyword phrases for you whenever you search anything... scroll down to the bottom below the search results, and you'll see about 8 related keyword phrases presented to you. Using Google Trends to compare those to the phrase you just searched will reveal a lot about which keyword phrases are used more, and which are used less.
Links. Links links links. Backlinks from valuable websites are key here. Getting other websites to talk about and link to your content is the vital element of off-site SEO. There are MANY ways to do this. Straight up contacting web admins of other sites, contributing articles to other sites, leaving comments are all ways to incorporate links back to your own website. And... guess what? Social media accounts are another way to get those backlinks. Your own social media, or other companies' social media.
Some "SEO experts" will simply list a link to your site on as many weird, useless websites as possible. Google doesn't really like that. Links from nytimes.com will be more valuable than links from spammy link sites.
See what I did? I just REVEALED ALL OF MY SEO TRICKS. There are more concepts and ideas in SEO, but above are the basics.
SEO is not black magic, it's just a set of actions you take on the internet, on your site and on other people's sites. Because so much of SEO revolves around the content on your site, it must be incorporated into your web development from the beginning.