Ball placement is a very important part of the game. Everything from serves to rally shots have an important element of ball placement.
There's an old saying:
"Amateurs just try to hit the table,
Intermediates aim for the corners,
Professionals aim for the extremes."
In the middle of a rally, aiming for the corners is a smart move as it can often catch your opponent out of position and unable to return the ball, but even better are the extremes. The extremes are closer to the net but at the side edge. Aiming here is much more effective than aiming at the corners.
The extreme angles are shown as the orange spots. The corners are the grey spots.
In addition to aiming for the extremes, you should also aim for the opponent's playing elbow (on the arm that they hold the racket). This cramps them so they can't easily make a strong return, requiring them to step around.
These are sub-pages of the Ball Placement page (a child in the hiararchy).