An ISyncRef is an Interface to a field that stores a reference to an IWorldElement. It uses the Data Model (FrooxEngine's synchronization structure). It is used in components such as the Reference Grab Receiver Component.
Sync References are a more abstract concept, but in simple terms they store a reference like a box.
In more complicated terms, ISyncRefs are a box that can store a pointer to another item, and allow for keeping references for later. For example, Reference Grab Receivers can use a reference to this component as a box to put the reference a user drops into the reference grab receiver. It can then store the reference for later reference in ProtoFlux.
Another example is Data Model Stores with object references, which act as a user friendly ProtoFlux version of using an ISyncRef field on a component. A user could use an ISyncRef as a Data Model Store, but this is very bad practice.