SERP is an abbreviation from Search Engine Results Page –  a webpage you get when a search engine returns your the results of your search.

For instance, this URL will open you a Google search engine results page for the SERP word:

On most SERPs you would see two types of search results: most relevant pages from the search engine database (these pages were indexed and according to the search engine's logic are most relevant to your query), and sponsored links – still relevant to your query resources, these are the results which have been paid for by their respective owners.

A typical search engine results page will contain a number of web pages links with their titles and a short description with optionally highlighted words which matched your search query.