It's the stuff they throw out during hunting season. Supposedly (I'm only sharing information I've read) that it's better to buy the extra clean rather than the basic feed corn because it has a tendency to be cleaner (no cobs, rocks, etc) and less chance of bugs.
Also...if you use corn...you need to run the insert part through 3 heating/cooling cycles to ensure that any extra water has evaporated, mold spores, bacteria, insect, etc that could cause damage down the line before putting it in the cover.
Again, don't know how accurate info is...however I have read it on several of the blogs & patterns I've looked at.
You can also use rice or flax seed. I opted for corn because I have one & like the weight. Plus it was an inexpensive option.