http://en.wikipedia.org/wiki/Sitemaps

The priority of that URL relative to other URLs on the site. This allows webmasters to suggest to crawlers which pages are considered more important.

The valid range is from 0.0 to 1.0, with 1.0 being the most important. The default value is 0.5.

Rating all pages on a site with a high priority does not affect search listings, as it is only used to suggest to the crawlers how important pages in the site are to one another.