It's entirely up to the JVM to determine what goes inside a reference.
In the simplest case it would just be a pointer (i.e. an address). In more sophisticated cases, the VM may use different representations depending on the situation - for example, you may want to read the paper on "Compressed oops in HotSpot" to see how the HotSpot VM can avoid doubling the size of references in some (but not all) places when running as a 64-bit VM.
The important thing is that you should neither know nor care. As far as you're concerned as a programmer, it's just an opaque set of bits - its only purpose is to let you get to the object in question (or identify a null reference).
