SEO is 'search engine optimization'. It's the practice of optimizing and making your site better to increase the quantity and the quality of the traffic to your website through organic search engine results.
What's important in SEO?
Keyword density is one thing you've probably heard about in regard to SEO. It means that your blog or page or post has a keyword. You need to use it enough times (density), but not too many that it becomes overwhelming. Make sure you use them thoroughly and frequently, an exact percentage like many claim isn't exactly right.
Links help determine how popular things on the internet are. You want to be trying to link build. This means having other sites link back to yours or a specific article/blog. Shares on social media also help in spreading links of your site around. This is largely how search engines like Google can help to rank the popularity of a site-the amount of links and links shared.
Functionality of your site
Your site needs to be able to function and run properly so that bots can 'crawl' it and index your content. This means no broken links, changed or missing URLs or even weird code. Everything needs to function so that bots are able to crawl your site properly and share this with search engines like Google.
Make sure your content is good
You want to ensure that you have good quality content. Make sure that it answers the question or addresses the title. You want it to be share-worthy and shareable content to ensure it's successful.