Drupal - What does this security update message mean?

Sometimes, when you edit your Drupal site, you may see a message like this one:

There are security updates available for one or more of your modules or themes. To ensure the security of your server, you should update immediately! See the available updates page for more information.

This is a warning from Drupal that a security update has been released and must be installed.

IT monitors all Drupal security update releases, and manages software updates centrally.  When an update is released, IT downloads and tests it, then applies it to all SSU Drupal sites, usually within 1-3 business days.  In most cases there is no noticeable downtime, and nothing department content owners and editors need to do about the update.