It is an idiom meaning a person has performed a particular action so many times they have mastered it to the point of perfection. They know exactly what they're doing and can precisely duplicate the results each time.
Idiomatically, ff something is referred to as a 'science', it is precise, regimented, and each step can be documented. If something is referred to as an 'art', then it's more hit or miss, intuitive and can't easily be explained. Each result may show show individuality, and may not be an exact duplicate of the previous attempt.