Spine tilt during your golf swing sets the angle that you will swing the club head back along.
This angle that the club head travels around you at, known as your swing plane, dictates whether you will strike clean, crisp golf shots or not. If the angle is too upright or steep the club head will have a too vertical or steep angle of attack. The club head will be moving back towards the ball too vertically and this will result in topped or thin golf shots. If the club head moves around you on too flat or low an angle then it will attack back towards the ball on too shallow an angle and the club head will either hit the ground too early, resulting in a fat shot that looses power and distance, or the club head will skim over the surface and catch the ball on the upswing producing a topped or thin shot.
To improve your ball striking, work on improving your spine tilt at set up. When you are creating your posture, or spine tilt, at set up you need to keep your back as straight as possible. If you hunch your shoulders over and create a C shape look in your back, you will find it difficult to rotate your upper body well during your backswing and you will tend to swing back using your arms too much as you are unable to rotate your upper body well. This will produce a vertical or too steep swing plane in the club head and this will cause you problems with connection with the ball as previously discussed. As you are unable to rotate your upper body well, you will also reduce the power in your golf swing and not hit the ball as far as you possibly could.
To create a straight starting position with your spine, place a club down your back so that the back of your head, the back of your shoulders and the bottom of your back are all in contact with the club. Keep these three points, the back of your head, the back of your shoulders and the bottom of your back in contact with the club and tilt your spine forward from your hips, maintaining contact with the club in the three positions discussed. Tilt forward towards the ball until you are in a position where your arms would be able to dangle vertically under your shoulders as you hold the golf club to address the golf ball. How much you actually need to tilt forward will depend upon how tall you are and which golf club you are using. The taller that you are the more you will tilt forward than someone who is smaller in height. The shorter the club that you are using, the more you will need to tilt forward than if you are using a longer club. But whatever your height or which club you are using, work on tilting forward so that your arms are able to hold the club vertically under your shoulders.
Once you have achieved this straight back position and you have tilted forward to allow your arms to dangle vertically under your shoulders, soften your knees to take the pressure from your hamstrings.
This is the correct spinal angle tilt to create and maintain during your golf swing, so work on rotating around this spinal angle and you will see a big improvement in the connection that you make with the golf ball.