A mahl stick is a stick or thin pole about a metre in length (three feet) with a ball-shape pad at one end, used as an aid in painting, particularly in oil painting. A mahl stick is useful when painting detail or when painting in a large area where the paint is still wet and you want to avoid touching the surface accidentally.
How is a mahl stick used? Rest the ball-end of the mahl stick on the edge of the canvas, on the easel or even on a spot of the painting that's dry. Hold the other end up with your non-painting hand and steady your arm holding the brush on the stick while you paint. If you rest the mahl stick on your little finger and forearm of your non-painting arm, you can (with a bit of practice) use the other fingers of that hand to hold your palette and extra brushes.
A mahl stick is something you can make yourself, from a piece of bamboo or dowel; simply tie a folded-up bit of cloth to the one end with a piece of string, or use an elastic band. Ideally you want a bit of cloth that won't slip or scratch the canvas; a piece of chamois leather is ideal. A round stick is better than a square as you never have to orientate it so you don't rest your arm on an edge.
Other DIY options for a mahl stick include a walking stick (hook the handle over the edge of a canvas) or an old golf club. It doesn't have to be a thing of beauty, just stiff and not too heavy.