My understanding is that "roll" is the same as "carry".  The word roll means to move the contract ahead.  The benefit is the difference between the delivery contract futures price and the month you chose to roll the contract ahead to.  I had a bid in with my elevator that if I could roll my fall corn off of Dec 2010 to July 2011 for .30 to roll it.  They did.  I have one on to roll fall corn to March for .16, it has not rolled yet. 

Cost for my elevator is .02 to roll.  They want to know 6 months ahead and it is still the elevator's option to roll or not.