Neutral Spine is a medical term that means "good posture." It refers to the 3 curves in the spine: When the spine is aligned properly, a person is able to keep good balance, optimize breathing, and allow good flow of body fluids.
"Posture" (Latin): to put or place
Note on the "Posture Man" below, the line going through the center of the body. When you "posture" think about a center line going down the middle of your body.
If you sit/stand correctly, your playing will improve 100%, and physical symptoms you may be having -- numbness, tingling, pain -- will also improve or go away completely.
When your body is out of postural alignment, bad things begin to happen, including joint dysfunction and nerve damage.
Here are some tips to help you sit/stand correctly with your instrument:

Cervical
Thoracic
Lumbar