In natural referencing, the Sitemap is part of an essential optimization base.
What exactly is this SEO sitemap?
Why is it essential for your site to be indexed by search engines, and more particularly Google?
A Sitemap,is an XML format file whose function is to list the different URLs of the same site.
Each of them is associated with a certain number of metadata:
the date of creation or update;
the frequency of modification of the content;
the relative importance of the url compared to the other urls of the site.
In addition to listing all the pages of a showcase site or an electronic store, an SEO Sitemap in XML format can also contain images, videos and many other files such as PDFs.
In this case, it may be a good idea to create a dedicated sitemap.xml to facilitate indexing by search engines.

Sitemap, standard web protocol since 2006
To exist on the web, any site needs to be referenced in search engines such as Google, Qwant or Bing.
They use crawlers, also called spiders or bots, to browse the internet, identify pages and documents on the web or even follow the links between sites.
This tedious work of collecting data essential to the indexing of the web involves ever increasing resources.
In 2006, for search engines, it becomes important to gain in efficiency and to index more quickly the millions of pages which are created every minute, while controlling the resources allocated to indexing.
The three leaders of the time, namely, Microsoft Live Search (the future Bing), Yahoo!
and Google then agree on the adoption of a standard, the Sitemap protocol.

The objective of a Sitemap is undoubtedly to facilitate the indexing of all the pages of a website.
This must also be submitted by the webmaster or the SEO agency to search engines through the Bing Webmaster Tools and the Google Search Console.
The SEO site map thus submitted provides them with important information, in particular as to the frequency of visits by spiders on a particular page.

Example: A Sitemap indicates that page 1 is modified daily, while page 2 is only modified once a year. Google allocating only resources and a limited time to its bot to crawl the site in question, the latter will therefore logically pass infrequently on page 2.

On the other hand, the fact of indicating that page 1 undergoes daily modifications does not mean that the crawler actually passes there every day.
In addition, you should not confuse the fact of being able to submit all the URLs of a site and the indexing of these same URLs by search engines. Submitting an XML file actually makes their job easier. However, this in no way guarantees the referencing of all the pages or images appearing in your site map.

Note that many errors when generating an SEO Sitemap can block or slow down the indexing of a site. Obsolete version of the XML standard used, pages inaccessible due to the robots.txt file or too many URLs listed are among the most common errors.
Today, the SEO Sitemap is a basic of natural referencing.
Submitting it to search engines has little influence on your positioning.
On the other hand, failing to meet this basic criterion or constructing your
sitemap poorly
can serve you much more.

The acronym SERP  for  Search Engine Results Page  simply designates a results page following a request made on a search engine. It is presented in the form of a succession of links each referring to pages and websites, one below the other. According to the relevance represented by each of these links in relation to the request launched, the sites and web pages are classified in descending order by the search engine: from the most relevant to the least relevant. The relevance of these sites is also influenced by SEO, which aims to  rank the sites  in order of importance.
However, according to the research carried out by the Internet user, these SERPs appear in different ways and offer more or less varied results (in terms of content and links).
Thus, on a results page, we can get different things: URLs or title tag (clickable element), a meta description (small description paragraph below the link), the website icon / logo , a selection of Google images when the search is made there, the type of document in the case where the resource is a document, the date of the resource and the author if it is an article, a link Google Maps, Youtube videos and others related to research, etc.