SEO is the abbreviation of the marketing term Search Engine Optimization. With SEO you optimize your website to achieve higher positions in search engines, for example to attract more visitors to your site.
A characteristic of SEO is that it is often a sustainable form of marketing, by investing time in SEO your product or website can benefit for years. This is of course dependent on competition.
With SEO you ensure that your website can be found organically, these are the (free) search results that come after the first (paid) ads. You do not have to pay anything for a click on your website in the organic results.
Search engine optimization is often part of an online marketing strategy and is often confused with SEA (Search Engine Advertising).