This template "clears" both margins; it is often used before a header to make sure that the header will be the full width of the page.
Usage
To clear both margins, type {{-}} on a new line. To clear the left margin only, leaving the right margin uncleared, type {{-|left}}. You may wish to only clear the left margin if there is an infobox to the right of the article and you do not wish to leave a large gap of white space to the left. To clear the right margin only, type {{-|right}}.