The banking offset basically defines where the centre of rotation for the track is.
+2 means it's above the track, 0 between the rails and -2 under the track. Important is what it does to the guests riding the coaster.
http://www.serenafate.com/img/planetcoaster/bankingoffset.jpg
On the left side with no offset the track is twisted around itself, but the center of gravity for the cars is above the track, so it not only rotates, but also shifts laterally and vertically as it moves in a circle around the track.
The middle with a positive offset looks more like barrel roll, the center of gravity is in or below the center of rotation resulting in little or no vertical/lateral shift. In a perfect barrel roll the resulting g-force is always downwards from the view of the guest, so they could actually pour a cup of coffee while riding the coaster through this roll.
Having the center of rotation below the track (right picture) results in a much more violent shift, the cup of coffee would fly through half the park. A roll with negative offset also has quite a lot of lateral g-force..
For turns themselves this has little or no meaning as the banking is already established and the resulting direction of g-force is mostly dependent on the angle of the banking in relation to the turning radius. But it's important for initiating the turn.
http://www.serenafate.com/img/planetcoaster/bankingoffset2.jpg
Left side has a positive offset resulting in a very smooth entry to the turn. If it's perfect the direction of g-force is again always downwards for the people riding the coaster with a small initial rotation just before the turn starts. The track itself looks then like it turns in the wrong direction at first.
The right picture looks smooth again, but is actually banked in the wrong direction. So it's very useful if such a turn is desired to have more excitement/fear in a ride (probably more nausea, too). The direction of g-force is therefore in the turn itself and in the initiation upwards from the guests' perspective, so again a smooth result.
Edit: So to initiate a correctly banked turn the offset should always be positive, for a wrongly banked one negative.