This will occur if the page uses SSL (uses a "https://" connection) but includes references/links to items that do not, i.e. use just a "http://" connection. If linking to files on your site, either be sure to use a "https://" connection, or use relative paths without the http part (e.g.: <img src="/images/image.gif"> or <script src="javascript.js" type="text/javascript">). If linking to off-site items, there may be no work-around if those sites do not provide SSL connections via https:// connections.