Roll is a Defense stat that aids you in resisting Throw attacks. A higher Roll value will reduce the damage you take from any Throw attack that hits you. It also helps you succeed at Blocking a Throw attack, and aids in Just Defended attempts against Throw type, assuming you have the Just Defended ability.
Several things can influence a character's Roll score. Smaller people tend to get out of joint-locks easier, and land softer when tossed around, so they have an inherent advantage in this category. Training in grapple or throw-based styles, such as Judo, Wrestling, Sumo, and so forth may also help improve someone's Roll score.
A higher than average Roll score is usually accompanied by a lower than average Soak score, or some other similar penalty. One of the more common arrangements you may find will be someone that is strong at Throw, Roll, and Reflex, but weak at Physical and Soak.