It is the belief that a team of people can achieve more than a single person going it alone. It is the realization that you don’t have to do everything yourself, be the best at everything yourself, or even know how to do everything yourself. Your job, as a manager, is to get better outcomes from a group of people working together. It’s from this simple definition that everything else flows.
You can be the smartest, most well-liked, most hardworking manager in the world, but if your team has a long-standing reputation for mediocre outcomes, then unfortunately you can’t objectively be considered a “great” manager. Never forget what you’re ultimately here to do: help your team achieve great outcomes.