DSC, Digital Selective Calling is a mechanism for calling a radio individually using the MMSI of the ship as a Phone number.
DSC can be very handy, for instance on a night passage its not unusual for a large container ship to ignore your calls to ascertain if they see you and are intending to take evasive action, BUT if you call them using DSC the call is logged and so they will answer it more often.
But more importantly, countries around the Mediterranean often broadcast a weather warning via an all ships DSC call and once your radio has auto recieved this your channel will be changed off CH16 to whatever channel they are using for the broadcast.
If you do not understand how to cancel the call, you will not be able to get your VHF back onto the emergency channel (CH16).