A mahl-stick also referred as maul-stick, is a long wooden stick used by painters as a support to keep the hand that holds the brush from touching the painting surface.
Some artist make their own homemade version with a wooden dowel that has a tennis ball attached to one end. When on the go, I personally favor a curtain rod, which allows me to make it longer and shorter according to need and fits neatly into my french easel box.
But for studio work and paintings that require a very steady hand my favorite tool is the mahlstick made by Ez Rest.EZ Rest Painting Handrest - Mahl Stick $39.99 Creative Mark
EZ Rest measures 30¾" long and rests on a mounting cube, which can be clamped anywhere on your easel. The handle is held by the base so you can rest your hand without touching the canvas surface. The forward pressure of the resting hand , locks the shaft in place and stops it from sliding up or down.
I prefer to use this particular tool, because it does not touch the canvas and it is less likely for my hand to slip and create a mess on an already wet canvas surface.