The page Google shows after a search. Your target for ranking.
The SERP is what Google returns for a query: 10 organic results, plus ads, plus features (featured snippet, people also ask, knowledge panel, images, video, maps). Modern SERPs are packed with features that steal clicks from traditional results. Understanding the SERP for YOUR target keyword tells you what ranking really means (position 1 might still be below a featured snippet or ad pack).
Before writing content for a keyword, look at the current SERP. What's ranking? What format? What angle? Your content has to be equal or better to have a chance. Don't write blind.