A sitemap is actually a map of your website.
When Google crawlers go around the web indexing new web pages in its database, new blogs are usually missed out as only a few or no sites are linking to them as well as they’re not aware of sitemap thing.
A sitemap is either an XML or HTML file that contains all the URLs of the pages inside your blog.
While HTML sitemap helps your visitors to navigate through your blog, an XML sitemap helps the search engine crawlers the same way.
When you submit an XML sitemap to Google Webmasters, it helps search engine crawlers to find all the URLs on your website and add it to their database.
There are various parts of a sitemap that affect the crawling rate of your blog, such as the size of your sitemap, the number of URLs in your sitemap, indexing priority of pages on your blog, updating frequency, etc.