They mean that you cannot setup the leads so that they recieve anything in exchange for completing the fields on the offer.
Just to elaborate, means that you cant give them cash, prizes, gifts, anything in return for completing the offer.
Elaborating further, there are some sites that do nothing BUT incentive offers. For example, the annoying flash ads you see, offering you a free gift card if you just complete a survey (and fill out an offer or two or ten), are incentives. A market has sprung up where people "trade" their offers...for example, I'll fill out an offer on YOUR site if you fill out an offer on mine. That market has become very saturated though and it's hard to make any kind of reasonable profit.
Well, back to drawing board and back to building quality content rich affiliate sites and driving traffic and getting paid per sale.