A trajectory is the path a projectile takes through the air. When shooting a basketball, the player must find the ideal trajectory where the ball avoids all obstacles, stays in bounds and goes in the basket.
You want your hand and elbow positioned underneath the basketball so you can push the ball in an upward arc allowing the ball to drop into the hoop instead of flying through the air in a flat trajectory.
You would make the basketball about 5 inches higher than the basketball hoop. Make sure you body is square to the hoop, your feet are shoulders width apart and your elbow doesn't go farther than a 90 degree angle. Recommend This Please.
A high arch is usually better than little arch but it depends on the players size and fundamentals or mechanics of the shot. A consistant trajectory is important in forming a consistant shot. This is true with other mechanics like release point, form, etc.