A reference object refers to a class that is implemented in such a way that all instances with the same value share properties. Thus changes to one instance of an object representing a user should also show up in another object representing the same user. Reference objects are mutable, in that their state can be changed without creating a new object.
Flourish includes a few different classes that are reference objects:
Here is another page with some info about reference objects:
The opposite of a reference object is a value object.