Cron is a operating system level function, which may be available for you to manage in your host panel of your server. It is basically a scheduler that can run programs on regular, scheduled times.... like every 10 minutes.
The first thing to do is to see if other scheduled jobs are running via cron. Go to Administer | System Settings | Scheduled Jobs. On the resulting page is a list of all the possible jobs that might run via cron. Note the column "Last Run"... if cron is running hopefully you will see a recent date/time shown in here.
If cron is running, then find the scheduled job entitled, "Send Scheduled Mailings". This is the job that actually will send your CiviMail job out. Make sure it is enabled. If not, enable it and see if things run.
If cron is not running, your best bet is to contact with your hosting company to have them check for you. Refer to the CiviCRM documenation for help in setting it up: http://wiki.civicrm.org/confluence/display/CRMDOC/Managing+Scheduled+Jobs