A while ago I heard a word that means something representing something abstract, but not actually being that thing. It was in the context of Java's Class objects - that classes in Java are loaded into the JVM in a special way, but that Class objects exist to represent those; they're _____. I feel like it started with an R? Thanks!
It sounds like you're thinking of reification. From Cambridge Dictionary:
noun [ U ] /ˌriː.ɪ.fɪˈkeɪ.ʃən/ /ˌriː.ɪ.fɪˈkeɪ.ʃən/ formal
the act of changing something abstract (= existing as a thought or idea) into something real:
the reification of fantasies
This standard definition is very slightly different from an existing object that stands in for something that remains abstract (that sounds a bit more like a symbol), but I can see it being used that way in a specialized context. I've mostly heard it used in the social sciences, but Wikipedia says that in computer science
Reification is the process by which an abstract idea about a computer program is turned into an explicit data model or other object created in a programming language. A computable/addressable object — a resource — is created in a system as a proxy for a non computable/addressable object.
That second sentence, especially, sounds like what you've described.