SERP stands for Search Engine Results Page. It is the page that you see after you type a query into Google, Bing, or any other search engine.
Modern SERPs are highly dynamic and feature much more than just traditional blue links. Depending on the search intent, a SERP may include paid ads (PPC), a Local Map Pack, Featured Snippets (quick answers at the top of the page), 'People Also Ask' accordions, image carousels, and video results. The primary goal of SEO is to secure the highest, most prominent position possible on the SERP for relevant keywords.