Select triangles within a circular area around the mouse pointer
The brush tool is used in all those places where triangles need to be selected in order to process them further. The tool helps by providing a circular area around the mouse pointer within which triangles are marked or unmarked at once, rather than having to select all the triangles individually. The circular area can be adjusted in size.
Working with the brush
When pointing at a triangle, the brush plane is angled to lie flat and parallel with that triangle. If the brush is larger than the mesh underneath it, the brush's area of effect will wrap around the part.
Depending on the module, there is also the option to account for the edge angle between two triangles: If the angle is beyond the threshold, ie, the edge being sharper than a given value in either direction, triangle selection will not bleed past the edge even if a triangle was intersecting the brush.

Use the brush tool to select multiple triangles at once. During repair, an edge angle threshold can be set to prevent selecting unwanted triangles.
To change the brush size, hold Ctrl while scrolling the mouse wheel.
Functions that make use of the brush
Repairing a mesh:
The semi-automatic repair and mesh editing actions, as well as simply deletion, apply to selected triangles.
Generating supports:
You can select triangles into clusters and then apply support structures to those clusters.
Finding desirable orientations of a part:
The estimated support requirement of a part's orientation is one of the criteria used in finding and ranking alternatives. You may mark triangles as "Do not support" to adjust the calculation and its results accordingly.
Painting a mesh with colors:
When applying a color to a mesh, any triangles which intersect at least partly with the brush circle will receive the color.
Manipulate a mesh:
The brush size determines which vertices will be affected by extrusion and distortion.