It is the address, to be precise, it is the value of the memory valueOffset position of this object compared with expect.
Detailed explanation of Unsafe

As the questioner said, AtomicInteger源码是比较并替换Integer来实现线程安全性。而AtomicReference is a comparison and replacement compared to object references. These are atomic class CAS implementations.

As for comparing addresses, let’s start with what the questioner said AtomicReference and know the following methods:

public final boolean compareAndSet(V expect, V update) {
    return unsafe.compareAndSwapObject(this, valueOffset, expect, update);
}

The underlying implementation is in Unsafe类中，是一个native本地方法。Unsafe的CAS包括了三个操作数--需要读写的内存位置valueOffset，进行比较的值expected，拟定写入的新值update. CAS atomically updates the old value of the memory address with the new value if and only if the value stored in memory location V is equal to the compared value A. Otherwise, no operation is performed.

The key lies in the role of the passed in valueOffset, continue to view valueOffset的作用，继续查看AtomicReferenceSource code:

static {
  try {
    valueOffset = unsafe.objectFieldOffset
        (AtomicReference.class.getDeclaredField("value"));
  } catch (Exception ex) { throw new Error(ex); }
}

private volatile V value;

Here, the unsafe的objectFieldOffset method is to get the memory offset of the object, that is, through comparison here, you can determine whether it is the same object address.

So, conclusion:

Atomic atomic CAS operation compares memory offsets, that is, memory addresses.