The Landscape tool's brush defines the size and shape of the area of the Landscape that will be affected by either sculpting or painting. Brushes can have different shapes, sizes, and falloffs. Brushes should be a familiar concept to anyone who has experience using Photoshop or a similar image-editing application.
You can set the brush type and falloff in either Sculpt or Paint mode.
This determines the size of the brush in Unreal Units including the falloff. Within this area, the brush will have at least some effect.
This determines the percentage from the brush's extents where the falloff should begin. Essentially, this determines how hard the brush's edges are. A falloff of 0.0 means the brush will have full effect throughout with hard edges. A falloff of 1.0 means the brush will only have full effect at its center, and the effect will be reduced throughout its entire area to the edge.
Use Clay Brush
If selected, uses a clay brush.
The size and falloff of the current brush is displayed in the viewport as a flashing, light-purple filled circle, or pair of concentric circles.