Unless you have a huge first serve or a blistering return of serve, your success as a tennis player is often going to be determined by the quality (pace & depth) of your neutral ball.
To me, a neutral ball is one that is not an obvious offensive position where you are equipped to end the point with a winner or even force your opponent into an error. It is also not a ball that puts you on flat-out defence where you are scrambling just to get the ball back in the court. A neutral ball is typically one that is just continuing the point without much drama. However, I think the ability to understand what phase of neutrality you are in will often determine your success as a tennis player.