It gets even more confusing when looking at monster stats, because they usually have someone rolling opposed Str or Dex to escape a monsters grapple maneuver.
Anyway, I'm pretty sure it's not entirely clarified in the rules, but I do remember seeing a large FAQ or interview somewhere about combat they might cover it.
IIRC, a maneuver can either give a bonus die or a penalty die, so for grappling I usually give a penalty die to the grappled. They basically can't do too much except try to escape or try to harm the grappler, both with the penalty die. When someone attacks them, they can fight back or dodge, but again with the penalty die. This is just how I do it, not necessarily correct.