i know little about sitemap.xml but i want to know more what purpose we use sitemap and how to i insert to my site ?
To help search engines index more pages faster. Here's some xml sitemap tutorials on how to create sitemaps, FTP upload (what you mean by "insert" I think) etc.
If your site is well designed, it probably won't add much, but I did recently get some images indexed that weren't otherwise discoverable by search engines. Announce your sitemap in robots.txt Sitemap: http://example.com/sitemap.xml Full URL. Google totally ignored mine until it was.
In a sitemap you place all of the links of your site that you want the Search Engine to know about and the pages that you want seen in the Search Engine results. These would normally be the pages that are very deep in your site that would otherwise be missed by the crawlers when they visit your site. You upload the sitemap in an XML format to the root directory of your site and then manually submit it to Search Engines like Google via Google Webmaster Tools.