from the google site map documentation : "Describes the priority of a URL relative to all the other URLs on the site. This priority can range from 1.0 (extremely important) to 0.1 (not important at all).
Note that the priority tag does not affect your site ranking in Google search results. Priority values are only considered relative to other pages on your site so, assigning a high priority (or specifying the same priority for all URLs) will not boost your entire site search ranking."