When a violinist prepares to play, you see them first tightening the bow, then applying a small object over the bow hair. A bow is shaped with a little bit of a curve. To maintain the curve shape, it has to be loosened each time it is not in use. You simply turn the knob at the end of the bow and then turn it the other way the next time you use it. The object that violinists apply over the hair of the bow is called the rosin. Bow hair is naturally slippery and does not make a sound on its own. It needs friction and sticky rosin triggers the friction. The rosin leaves a residue on the strings and it has to be cleaned off.