Rosin coats the hair of the bow, causing it to grip to the string. The bow drags the string, displacing it from it's position of rest before eventually, tension overcomes the friction and the string slips back. This continuous cycle of of displacement sets up a vibration in the body of the instrument - this then radiates through the air as sound.
Finding the right string height or action for your violin can be a delicate process: a balancing act between the standard measurements that are used by luthiers, the unique characteristics of the violin in question, the strings being used, and the particular tastes of the musician. Once addressed and correctly set up though, the action on a stringed instrument can have an enormous effect on not only the playability and comfort of the instrument, but also the sound.
The bridge on a stringed instrument acts as a resonator for the instrument, capturing the vibrations from the strings and transmitting them to the body of the instrument, allowing it to resonate throughout and to turn into the wonderful big sound that our ears know and love. For this reason, the bridge is a vital part of your instrument that must be paid attention to, and taken care of just as much as any other part of the instrument.