Your Apple ID is your link to Apple and is plugged into every service from iTunes to the Appstore.
Recently reports are coming in of Apple ID accounts being hacked.
Details are sketchy because this is very new, but it seems that those affective have been reporting odd messages appearing in their phone (all written in Chinese) and notifications of foreign devices gaining access to the Apple ID.
Apple have posted a number of guides to help those affected but the bottom line is, change your Apple ID password and turn on two step authentication. Something that really should have been turned on since day one.
So as not to re-invent the wheel, here are some guides that will help.
Then, two step verification.
Finally, a general guide form Apple on Apple ID management.
Good luck and, as always, if you need help please call to arrange a call out.