It means you will be working in an office. As it is work experience you will probably be doing the more mundane tasks, such as opening post, answering the phone, filing, photocopying and things like that.
Other duties will probably depend on the type of place you are going to be working in.
As Cheeky says, it�ll be the more menial tasks but it will give you an idea of a working enviroment even if it isn�t where you see yourself in a few years.
I�d also say don�t view it as a �holiday� from school, take on board what you�re taught, attack it with enthusiasm (however mundane) and aim too impress - whoever you�re working for could be used as a valuable reference in the future.