This means there is a being with a mental model of the relationship between the symbol, and what it stands for. The easiest thing to do with a symbol is to copy it, and there may be arbitrarily many copies of a symbol all with exactly the same reference. Furthermore, the same symbol may be used over again in a different context with a different reference, but within the same context, the copies of a symbol have the same reference. Although the flow of symbols may dazzle the eye, what matters is the mental model that choreographs their dance. A computer is merely an elaborate equivalent of pencil and paper for performing symbol algorithms on the behalf of human beings.
Physical algorithms move things around.
You can always tell when a physical algorithm is being performed. Cooking a meal or building a skyscraper are the same except for scale in terms of the number of forces applied, the number of physical objects to which they are applied, and the time over which the activities are completed. They contribute to the understanding of all the physical activities of beings, including physics experiments. Mental algorithms direct attention. You cannot see the performance of a mental algorithm.