The term roller refers to the police, police car, or a patrol officer. This term was commonly used in the 1950's by white youths to alert their friends that a cop was rolling by.
I piece of graffiti (usually very large) done with a paint roller and bucket paint. Rollerbombs are made to get one's name up very large for a larger audience to view it, sometimes they can be done in block letters, can resemble the graffiti artist's throwup or may be many different colors. Sometimes smaller scale roller bombs may be outlined with spray paint to save time.