A Semantic device is a structure that is capable of manipulating values and attaching said values to deeper concepts. The most obvious expample of such a structure is the human brain. Compare this to a Syntactic device, which is ony capable of manipulating values and can never hope to grasp the context of the values.
Avout such as Orolo and Fraa Jad believe that the capabilities of a semantic device to seemingly build detailed models of the universe, then fast forward and reverse time within the model to explore consequences are made possible by the device's ability to read feedback from exra-universal versions of itself.