In One Sentence
SEO (Search Engine Optimization) is the umbrella term for tactics that get your site to rank high in the results of search engines such as Google and Bing.
What does this look like in practice?
For example, when a user searches "Tokyo delicious ramen" on Google, SEO is about getting your site to appear in the top 10 listings on the first results page.
The higher you rank, the more likely you are to be clicked (roughly 30% click-through for #1, around 2% for #10), driving more traffic to your site.