iCal 1.5.4 displays an "unsafe" alarm alert when you try to import a calendar that contains existing alarms that open documents or other applications.
If you are concerned about security, either be sure to remove all unsafe alarms during the import process or don't import calendars that contain these kinds of alarms.
The alert allows you to choose whether or not you want to continue importing the calendar and its events, including all alarms. If you choose not to, click "Cancel" to stop the import process.
If you want to import everything, click "Keep All Alarms," and iCal will import all events and their respective alarms, including ones that open other documents and applications. Keep in mind that this is the least secure option.
If you want to import the calendar without the potential security issues, click "Remove Unsafe Alarms." iCal will import all events but won't import any alarms that are set to open documents or applications. It will import all event alarms that play a sound or display a message.