A paper prototype study is a study conducted on a paper version of a design to get feedback early on in the design process.
Paper prototype studies are good for getting feedback early on in the design process to help confirm whether you are headed in the right direction. You can determine appropriateness of the concept design before too much work has been done and before the project team becomes tied to a particular design. They prevent costly mistakes once development has begun.
One disadvantage is the fidelity of the interface. Participants may be less likely to realistically engage with something on paper, however, they may be more likely to provide feedback because it is clear that it is in the early stages. It may also be difficult to simulate complex interactions on paper.