A media alert is an alert that is issued to the press about a particular event that's going on. So the media alert is basically the who, what, where, when and why of the event, and it has all that information, who's going to be there, what celebrities, what's going on. And that gets issued to TV producers, paparazzi, celebrity weeklies, all the newspapers, the photo desks, and everyone that needs to know what's going on.
You're basically alerting the media with this alert about "Hey, this event's going on and hey, these are the celebrities and this is why you need to cover it and this is why you need to write about it." This piece of paper is basically the bible with all the information that the media needs to know, so if they're going to write about your client or your event or whatever, it's all on that paper. All the facts are there, all of the everything is there. It's basically a go-to of information for the media to use, and also to get them to want to cover your event.