The IMC rating is an additional rating that can be added to your licence after completing the JAA PPL. The course covers flying in cloud using the instruments, instrument approaches, tracking radio aids and partial panel techniques.
If a lot of your flying further afield it can help open up days to you which would previously have been unavailable, it can be a good introduction to instrument flying before starting training for the instrument rating (which is required to fly commercially) or alternatively used as a 'get you out of trouble rating' if the weather suddenly deteriorates en-route while flying visually.