SEO stands for Search Engine Optimization, and is a general umbrella for a myriad of professions, techniques, concepts and practices that focus on the promotion of websites in search engine results pages. In other words - the practice of getting a website up to the first page in Google
It turns out that the vast majority of clicks that search results get go to the 1st, 2nd and 3rd results. The rest of the first page gets bread crumbs. Not to mention the 2nd page, which hardly anyone gets to. So if you want your website to get traffic (and ca$h), you better get it to the upper first page in Google.