Rosin is a sticky substance applied to the bow in order to generate a sound when bowed across the strings of the instrument. It's naturally made from resin and can be made in a number of ways, everything from novelty fun blocks such as pizza to handcrafted masterpieces.
The colour of a rosin indicates the tone it offers. With lighter rosin creating a brighter, sweeter tone and darker rosin creates a richer, warmer sound. A perfect beginner's rosin would be the 3V/C Hidersine rosin (V for violin, C for cello)