After years of research and discussion on edge technologies, the importance of Fog Computing for the Internet of Things (IoT) is not hidden from anyone. This technology serves at the closest distance to the operating environment and has a multi layered architecture that penetrates deep into the network. Many resource constraint and heterogeneous IoT devices send their requests to the Fog. The variety of equipment in this layer is remarkable, but the most important of them is Nano Datacenters. These nodes are located a hop away from the operating environment and provide processing, data storage, and communication services. Fog Nano Datacenters have beneficial features that enhance the ability of developers and researchers to properly design service delivery for IoT real time applications. In this paper, we tried to introduce them comprehensively, remove ambiguities and describe its services and values. Finally, we presented the challenges and remarks for future research directions in this context.