Hardened sap from trees which is used on violin bows, Bull Rider and Baseball gloves, bottoms of pointe shoes, etc. Helps retain a good grip on something, makes it not so slippery.
Dancer 1 : "Yesterday I was dancing in pointe class...and I slipped and fell on my butt..it was extremely embarrassing."
Dancer 2: "Did you get rosin from the rosin box?"
Dancer 1: "Oops, no."
Dancer 2: "Well that explains it."