Search engine optimization (SEO) is the work of making your pages easy for search engines to use and worth showing. It has three broad parts. Technical work makes sure pages can be crawled and indexed (technical SEO). Content work makes sure pages match what people are looking for (search intent). Authority work is about earning trust, including links from other sites.
Day to day it includes writing clear titles and descriptions (title tag, meta description), using headings well (H1), linking pages together (internal linking) and fixing problems that stop pages from being indexing candidates.
No one controls search ranking, and search engines change their systems, so be cautious of promises. The steady approach is to make pages genuinely useful, keep the site technically healthy and measure results in your search analytics.
Related: Crawling, Answer Engine Optimization.