Aside from what Azure Service Fabric actually is, its significance in my opinion is the fact that it provides a layer of abstraction over the management of the underlying infrastructure to run the application for developers.
This means that the developers do not need to worry about building an environment that is reliable, scalable, redundant, flexible, etc for their application. Azure Service Fabric does that for them and enables them to test and run their application on their local machine with a press of F5 and in an exactly same way that the application runs in the cloud.