What exactly does it mean when you have a headache for no apparent reason?

It is common. Headaches are very common as most of us have them.The causes are varied. It is often due to muscle tension or stress. You can use tylenol (acetaminophen) to help , if this does not work, you should see a neurologist or headache specialist to make sure it is not migraine which will require specific migraine medication(called triptans). Stress reduction will help. This requires exercise, slow down a bit and simplify.