A search engine results Page (SERP) is a page which turns out when users search something online using a search engine such as Google or Bing. Results are divided into organic and paid: organic results are relevant pages which are supposed to give a good answer to the user’s search, paid results are text-based ads, typically displayed on top and on the right column.
A wide range of factors influence the list of results and their order. In fact, today SERPs are highly customized for each user and for each use-case, taking into consideration not only the keywords of each query but also the context (with signals such as previous searches, location, time of the day, and much more).
The process of improving a website in order to gain better results in one or more SERPs is called SEO.