What actually are ad hoc problems?

Hello ! Incredible work at analyzing what kinds of algorithms/structures are most prevalent in tests. About ad-hoc problems, the description in the site is really vague. Though I understand that these problems are “simple” compared to the other categories, would it be possible for you to say more about them ? Can I find somewhere a list of problems of this kind (so I can explore the range of algorithms required for them, like what kind of array operations, what kind of string operations, etc.) ? Thanks =D