A sitemap is a map of a website, a very useful tool providing Google with all the informations about the website itself: a very important activity for the indexing of contents on search engines.
Technically speaking, a sitemap is an XML file that you can create from the CMS you're using. In addition to CMS, there are also free softwares that can easily and quickly generate sitemaps: we suggest you, for example, XML Sitemaps.
Once you create the sitemap, you have to report it to Google, using the Search Console, typing all the coordinates about the website and the sitemap itself. During the scan, the Google spider will record all the differences between the contents of the sitemap by the time of registration and the current ones. In this way, the search engine will process a new scan of the new contents to complete a new indexing.