Modifiers
The available brushes support several modifiers.
Place
Place new voxels in your volume.
Erase
Erase voxels from your volume. There is a shortcut when you are in the other modes to delete voxels: Press the shift key while clicking the left mouse button (at least in the default binding).
Override
Override existing voxels with a new color - but also place new voxels.
Paint
Other than override this modifier only changes the color of existing voxels.