SERPs contain two types of content: organic and paid search results.
When showing the search engine results, four paid search ads appear at the top on the page. All organic search results are shown below. Depending on the search engine and terms searched, there may also be snippets, images, video, or location-specific results shown.
The goal of most optimizers -- especially those concerned with Search Engine Optimization (SEO) -- is to get their website, or landing page, to rank 1st on the SERP. Or, at the very least on the first page of the search results.
At GuessTheTest, we like to say that ranking #1 in SERP means you've optimized your Search Engine Rank Performance.