Actions - What are objects and context
If you are talking about the $context variable (and I think you do), then the official description is: an associative array containing extra information about what triggered the action call, with $context['hook'] giving the name of the hook that resulted in this call to actions_do() (from here:  http://api.drupal.org/api/drupal/includes!actions.inc/function/actions_do/7)

But if you want to understand it better, this page is very useful: http://www.sysarchitects.com/node/47