What’s the important of Sitemap Attributes

Hi All Dp Member,
Here I mentioned the sitemap coding that i found from xml sitemap creator. but i am confuse that what's the important of these attribute in sitemap.xml page. and how to use or optimize for our sitemap.xml page. please tell me
<url>
<loc>/</loc>
<lastmod>2013-08-27T10:34:08+00:00</lastmod>
<changefreq>weekly</changefreq>
<priority>1.00</priority>
</url>
1. Here why do we use the <changefreq> tab
2. and also why we do we use the <priority> tab
Thanks.

See original post by seoteamvo