Mastery involves developing outstanding skill or expertise; having command of something.
What mastery do you have, or want to have, as a manager? Could it be a technical skill, an interpersonal skill, a management skill, a marketing skill? An interesting consideration, once you set a direction towards mastery, is where that skill or expertise will have value. Does your organization value that skill or expertise? Is it a skill or expertise you personally value? What is your purpose in developing that particular form of mastery? Where do you think it will bring you?
The journey to mastery is as valuable as its achievement. You will create focus, fine tune a skill and grow in the process.