Search Engine Optimisation, or SEO, is technically a marketing tool rather than a form of marketing in itself. The balance defines it as “the art and science of making web pages attractive to search engines.” The "art and science" part of SEO is what’s most important.
SEO is a science because it requires you to research and weigh different contributing factors to achieve the highest possible ranking. Today, the most important elements to consider when optimising a web page include: quality of content, level of user engagement, mobile-friendliness, number and quality of inbound links.
The strategic use of these factors makes SEO a science, but the unpredictability involved makes it an art. In SEO, there's no quantifiable rubric or consistent rule for ranking highly. Google changes its algorithm almost constantly, so it's impossible to make exact predictions. What you can do is closely monitor your page's performance and make adjustments accordingly.