Rosin is solidified resin taken from pines and other plants and trees, mostly conifers. It's made by heating liquid resin to evaporate the liquid.
In music, it's used in blocks or cakes by string players, who rub it on their bow hair to help make their instruments' strings vibrate clearly. Some types of rosin are mixed with gold, iron, silver, tin or beeswax to modify the friction and (possibly) the tone produced.
A selection of M&V articles about rosin
Ask Alice - On marketing rosin, with classical music agony aunt Alice McVeigh
Ask Alice - On Royal-Wedding-itis, cello rosin and a thumbs up for Obama, with classical music agony aunt Alice McVeigh