Creatine kinase is an enzyme or chemical that is released into the bloodstream when there is damage to the muscle.
That can be any muscle, including the heart muscle.
The most common injury to the muscle is physical trauma or over-exercising.
Muscles do not always suffer due to a physical injury. Some viruses, chemicals, medications, drugs of addiction, hypothermia can also cause muscle damage. Muscle damage is a rather serious process causing all sorts of complications including kidney failure, fatigue, palpitations, shortness of breath etc.
High levels of Creatine kinase or CK indicate muscle damage somewhere in the body.