Tiller has three meanings as we use it.
First is the measure of the braced bow for the largest distance from the string to the belly above and below the handle - this is usually at the tip of the fades. Most split finger shooters like 1/8 to 3/16" positive tiller - that is more space at the upper limb.
The second meaning is the curve of the limb throughout the draw, especially as viewed at full draw. There are different shapes like arc of a circle and arc of an ellipse.
Third meaning is a verb - the action of removing material to make the desired shapes above. This is the heart of the bowyers art.
Many bows have the upper limb some amount longer than the lower. This helps with the positive tiller.