A sitemap.xml file is a document that lists all the pages on a website and provides information about each page, such as when it was last updated and how often it is changed. This file helps search engines like Google to crawl and index the website more efficiently.

Here is an example of what a sitemap.xml file may look like:

```xml
<?xml version="1.0" encoding="UTF-8"?>
<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
   <url>
      <loc>https://example.com/page1</loc>
      <lastmod>2021-01-01</lastmod>
      <changefreq>weekly</changefreq>
      <priority>0.8</priority>
   </url>
   <url>
      <loc>https://example.com/page2</loc>
      <lastmod>2021-03-15</lastmod>
      <changefreq>monthly</changefreq>
      <priority>0.6</priority>
   </url>
</urlset>
```

In this example, the sitemap.xml file includes two URLs (page1 and page2) with information about their last modification date, change frequency, and priority. This information helps search engines understand the structure of the website and prioritize crawling important pages.