also see the faq entry how to compile vkdt such that you can use your pentablet as pressure-sensitive input:
you need a specific version of glfw to support it.
you can for instance clone https://github.com/hanatos/glfw,
for instance to /home/you/vc/glfw, and then put the
following in your custom bin/config.mk:
VKDT_GLFW_CFLAGS=-I/home/you/vc/glfw/include/
VKDT_GLFW_LDFLAGS=/home/you/vc/glfw/build/src/libglfw3.a
VKDT_USE_PENTABLET=1
export VKDT_USE_PENTABLET
of course you can also just use your mouse to draw masks.
for a description how to draw and how the strokes are adapted to the image,
see the draw module documentation.
to quickly add a draw module and the wiring around it to your image graph,
insert the draw block (e.g. by pressing ctrl-b in darkroom mode and select
the draw block, then insert it before a certain module in the graph).
drawn masks are especially useful in conjunction with retouching and inpainting. it can also be required to not only change the exposure of a masked region, but also grade it differently to match the surroundings when removing a shadow, for instance.
also see the (simpler) graduated density module which can create a simple linear gradient mask without brush strokes.