Role play, like bondage, is one of the most common forms of BDSM play. "Role play" refers to acting out a particular personality or scenario; in BDSM it is usually a sexual fantasy or some of the submissive actions the slave may do.
At it's most subtle and simple, role play can be the slave showing her most submissive nature, or exaggerating the arousal of a vibrator so her Master or Mistress can enjoy their role.
More often, role play is a particular scenario planned by the Master or Mistress where their slave pretends to have a specific job or emotional context. Do you want a naughty nurse that is trying to seduce you? Perhaps a shy schoolgirl? Perhaps your slave is dressed as a police woman, so you can show her who the boss really is. Maybe you are a Master that wants nothing less than to be worshiped as a god while you slave grovels at your feet.
Role playing is very common, and can be great fun for both the slave and the Master or Mistress.