what is mean of SERP in SEO ?

[online training]

There are three main types of results on a SERP: Pages that the search engine spider has crawled and indexed; pages that have been manually added to the search engine’s directory; and pages that appear as a result of paid inclusion. The highest-ranking hits generally link to the most useful information; links grow less relevant as they move farther down the list.

Depending on the number of Web pages that contain a particular word or phrase, a SERP might show anywhere from zero (in the case of no matches at all) to millions of items. For example, entering the phrase "complex-number admittance" into the Google search engine yields few results. In contrast, entering the single word "hurricane" yields millions of results.

In most cases, search engine users will look at only the first one to three pages of hits (results). Web designers and site owners use search engine optimization (SEO) methods to make their sites and pages appear at or near the top of a SERP.