With the Contributor's, you give Oracle permission to relicense your contributions to Oracle's projects. Download the PDF here:Oracle Contributor's Agreement (PDF file)
Please scan the signed agreement and mail it along with a hint that it is for project VirtualBox to: oracle-ca_us at oracle dot com (spam-protected).
There is a FAQ available and a list of signatories. Please follow this link for more information. The specification of a project speeds up the processing. You can specify multiple projects if you think you'll contribute to them, or update the list of projects using the same address.