What he meant by that is that the team that wins is often the one that makes the fewest mistakes in a game. Softball can be a game of mistakes. Hitters make mistakes when they swing at pitches out of the strike zone. Fielders make mistakes when they do not correctly read the situation and throw to the wrong base or forget to back up a teammate.
Base runners make mistakes when they mentally fall asleep and get picked off.
The team who wins is usually the team that makes the least amount of mental and physical mistakes. The trouble with mistakes is when there is one, there are usually many more just waiting to happen. Players often make multiple mistakes because they have never let go of the first one. They dwell on the frustration of the first mistake and their inability to let it go ends up distracting their focus away from the present play. As a coach it is important to remember that a player's focus and confidence are fragile following a mistake.