tfBrush operating in tfGrid, on the left, and tfWarp, on the right.
This slider increases and decreases the radius of the Brush.
The radius can also be adjusted by holding down the middle button on your mouse and dragging horizontally.
Set at 50%, by default, this can be reduced to give a lighter touch or increased to give a heavier touch to the deformation.
The strength can also be adjusted by holding down the middle button on your mouse and dragging vertically.
Depending on how you access tfBrush, the default setting will be Elevate in tfGrid, or Brush in tfWarp.
The other settings in tfGrid are: Off, Smooth, Set and Transfer.
In tfWarp, the other settings available are: Pinch, Bulge and Relax.