The phrase "management is an activity" simply refers to the various responsibilities that come with the assumption of a position in management. While the ability to hire the right people and allow them to perform their responsibilities without the temptation to micromanage them is an important component of managing, in most cases a more active philosophy of management is required. That does mean micromanaging; it does mean remaining engaged in the productive processes at the heart of the enterprise and representing accountability when things go wrong.
The responsibilities of management vary from activity to activity, but generally involve establishing goals and priorities, assigning employees to individual tasks, conducting some level of oversight of those employees as they carry out their responsibilities, and holding each employee accountability for the proper execution of his or her assignment. The "activities" associated with management include production and distribution of information pertinent to the tasks at hand, making decisions when required, ensuring that each employee remains engaged, and arbitrating the inevitable disputes that arise among and between employees and teams. An essential component of management is the "human element," in effect, recognition that employees are individuals with personal and professional problems that need to be handled sensitively but firmly and fairly. All of this is what is meant by the phrase "management is an activity." Management is not assigning work and retiring to one's office for the day with minimal or no interaction with subordinate staff. It is a dynamic process that requires innovative thinking and superior communication skills, both with respect to customers and to one's employees.