For email submits, it means scrubbing an email address against their master list. If they already have that email, you don't get paid. It's basically making sure there are no duplicates and in some cases nothing from the merchants blacklist, (though few admit it). It has other forms as well, but that's the basic idea.
It's almost impossible to avoid, since you don't have access to the list they're scrubbing against. It's not very hard to tell if they're getting really aggressive with it because your leads will be down for no apparent reason.