SERP is an acronym that stands for search engine results page. It is a term often used within SEO circles to describe the results that a search engine returns after a keyword phrase has been searched.
The most popular search engines (Google, Bing, & Yahoo) will typically return two types of results on a SERP, organic and paid.
The organic results of a SERP will display the most popular pages as determined by the search engine’s organic algorithm. In most cases the organic results will be sandwiched in between various sets of paid listings.
A SERP can vary drastically depending on the query and the query location.