These are optional tags of a sitemap protocol. Keep in mind that you’re only giving hints and not directions for action. It is not guaranteed that the search engines will do exactly as you have specified in these tags.
changefreq — How often the contents of the webpage may change
lastmod — The date of the latest file updates.
priority — Used to tell the search robots which pages of your website are more important than others and more desirable to be used in the search rating. Possible values vary from 0.0 to 1.0, where 0.5 is considered a default value.