At the simplest, a trespass fee is a charge to access property on which to hunt. It's usually a per day, per hunter (or gun) fee. Trespass fees are common in places like South Dakota where many hunters are there to freelance, or hunt on their own, without the assistance of an outfitter. They travel to the area to hunt without a plan or knowing exactly where they will be hunting and pay trespass fees to landowners for the privilege of hunting. In other parts of the country, trespass fees are sometimes called "Day Leases."
Some outfitters incorporate day leases or land requiring trespass fees into their hunting operation. That's why it's important to ask these questions of any potential outfitter ahead of booking your hunt. Since operations (usually individual landowners) who charge trespass fees seldom provide guiding, meals, accommodations, gear or equipment, or game care/handling they fall outside of the OutfittersRating.com system. However, sometimes outfitters will incorporate the use of day leases into their services. This is especially true when it comes to waterfowl guides/outfitters who are following birds to set up in preferred feeding fields. As these fields change so often, it's usually not practical to lock up such a large area with season long leases. Some big game operations also utilize land on which a trespass fee must be paid.
It's important to ask your outfitter if he/she utilizes these kinds of land and hunting opportunities in their business. If they do, that's fine, but establish up front, in writing who is responsible for paying the trespass fees. Are they built into the package price of the hunt or is it an extra that you'll be responsible for should you decide to hunt on this kind of property?