Think of it as a bulk submission directly to Google of all the pages on your site, updated every time you create a new one. Google's definition:
A sitemap is a file where you can list the web pages of your site to tell Google and other search engines about the organization of your site content. Search engine web crawlers like Googlebot read this file to more intelligently crawl your site.
There are a few plugins that can create a Sitemap file for your site. But the most popular by far is the multi-purpose Yoast SEO Plugin (free version).
Once Yoast SEO has been installed and activated, you can find your Sitemap settings at Dashboard menu/SEO/Sitemaps XML. See screenshot below.
The URL will usually be something like 'example.com/sitemap_index.xml'
As with any other issue you find on your site, you can contact our support team by emailing support@performancefoundry.com.
Log in to your Google Webmaster Tools and select the site you want to submit. Once selected, go to Crawl/Sitemaps and then click on the red button "Add/Test Sitemap" .