I think of it as too much supervision. A micro-manager might as well do the work himself (or herself.)
A manager should be specifying the goals or objectives and letting the employees get on with the work. If the employee needs clarification or direction the manager should be available to provide answers. And leadership. If the employee can't do the job without the manager then they haven't been adequately trained or they are the wrong person for the job.
More thoughts to follow....
It refers to a manager telling an employee how to do every little move they make.
It goes past making sure a worker does their job safely and efficiently, but to the point of telling them how to breathe.
In my personal experience, I have had only one past employer that was a micro-manager. He has the distinct trait of telling people how to do things, but could not do it himself. An expert in his own mind.