It's just what is on the land access permission slip that the DNR publishes. In IN you are to have WRITTEN permission on you at all times while accessing private land. That really isn't enforced, but the form as a whole is a pretty good one as it has lots of good information for the hunter and landowner to have. I used one when I was getting permission to waterfowl hunt - it seemed to sway some of those that may have been on the fence as it showed I was prepared and serious about doing things the right way.
The just of it is simple - your doing normal outdoor things on private land and something happens the land owner isn't liable - unless proven otherwise. IN has less than 3% public ground so this was used to help ensure folks would be able to have access to private ground. Some folks still use "fear of being sued" as a reason for not granting access - it's an excuse, but you can't force someone to allow you access.